LONDON – With England's capital city reeling from Thursday's terrorist attacks on its public transportation system, the focus turned to the hunt for who was responsible.
An organization calling itself the "Secret Group of Al Qaeda's Jihad in Europe" (search) claimed it carried out the coordinated series of four explosions, three on the subway and one on a double-decker bus during the morning rush hour.
That claim couldn't immediately be confirmed.
At least 37 people were reported dead and more than 700 were wounded in the worst attack on London since World War II, according to U.S. officials.
"It was chaos," said Gary Lewis, 32, who was evacuated from a subway train at King's Cross station. "The one haunting image was someone whose face was totally black and pouring with blood."
At least two Americans were among the wounded, U.S. State Department spokesman Tom Casey said, though their injuries were not life-threatening. There were unconfirmed reports that two more Americans had also been hurt in the attacks.
U.S. authorities confirmed a report that British police found two unexploded bombs in London after the morning attacks. Authorities investigating the blasts also recovered parts of explosive timing devices from some of the wreckage sites, FOX News has confirmed. The evidence should help in the search for who was behind the bombings.
Earlier reports suggested a homicide bomber may have been involved in the bus attack, though authorities later indicated they believed all four explosions were detonated by timing devices. But police were still investigating the possible involvement of homicide bombers.
The four blasts all went off within an hour, beginning at 8:51 a.m. (3:51 a.m. EDT). Authorities immediately shut down the subway and bus lines that log 8.4 million passenger trips every weekday.
British Foreign Secretary Jack Straw (search) said the explosions have the "hallmarks of an Al Qaeda-related attack." He added that neither Britain's police nor the intelligence services had any warning of the attacks.
"This is a very sad day for the British people," Blair said in an afternoon address from 10 Downing Street. "We will not be terrorized."
The bombings came a day after London celebrated winning its bid to host the 2012 Olympic Games (search).
"This clearly was a callous attack on purely innocent members of the public, deliberately designed to kill and injure innocent members of the public," Deputy Assistant Commissioner Brian Paddick of the Metropolitan Police (search) said.
The attack on London brought out a steeliness that recalled Britain under the blitz of German bombers in World War II, when many Londoners sought refuge in the Underground, site of Thursday's carnage.
It was an attack that Britain had long feared, following Al Qaeda's Sept. 11, 2001, strikes in New York and Washington and Britain's subsequent alliance with U.S. forces in Afghanistan and Iraq. Thursday's explosions also recalled the March 11, 2004, terrorist bombs that killed 191 people on four commuter trains in Madrid, at a time when Spain was part of the U.S.-led coalition in Iraq.
Police said the blasts at three subway stations went off within 26 minutes, starting at 8:51 a.m. in an Underground train just outside the financial district. Authorities initially blamed a power surge but realized it was a terror attack after the bus bombing near the British Museum at 9:47 a.m. — less than an hour after the first explosion.
Trapped passengers in the Underground railway threw themselves on the floor, some sobbing. As subway cars quickly filled with smoke, people used their umbrellas to try to break the windows so that they could get air. Passengers emerged from the Underground covered with blood and soot. On the street, in a light rain, buses ferried the wounded, and medics used a hotel as a hospital.
"I didn't hear anything, just a flash of light, people screaming, no thoughts of what it was. I just had to get out of the train," said subway passenger Chris Randall, 28, who was hospitalized with cuts and burns to the face, the legs and hands.
Rescue workers, police and ordinary citizens streamed into the streets to help. At the scene of several blasts, specialist emergency workers in orange hazardous-materials suits searched for evidence of biological, chemical or nuclear agents.
The bus explosion seemed to go off at the back of the vehicle, said bystander Raj Mattoo, 35. "The roof flew off and went up about 10 meters (30 feet). It then floated back down," he said.
"There were obviously people badly injured. A parking attendant said he thought a piece of human flesh had landed on his arm."
Doctors from the nearby British Medical Association rushed into the street to treat the wounded from the bus. "The front of BMA house was completely splattered with blood and not much of the bus was left," Dr. Laurence Buckman said.
At the request of Queen Elizabeth II, the Union Jack flying over Buckingham Palace was lowered to half-staff.
Blair gave a brief televised address from the G-8 summit taking place in Scotland before leaving for London. He concluded the attacks were terrorist actions.
"It's important ... that those engaged in terrorism realize that our determination to defend our values and our way of life is greater than their determination to cause death and destruction to innocent people and a desire to impose extremism on the world," a clearly shaken Blair told the world.
"Whatever they do, it is our determination that they will never succeed destroying what we hold dear in this country and in other civilizations in the world."
Later, at Downing Street, Blair said acts by Islamic terrorists should not reflect negatively on Britain's large Muslim population.
"We know that these people act in the name of Islam, but we also know that the vast and overwhelming majority of Muslims, here and abroad, are decent and law-abiding people who abhor this act of terrorism every bit as we do," he said in London after meeting with emergency-response officials.
"When they try to intimidate us, we will not be intimidated … we will not be changed," Blair added. "When they try to divide our people or weaken our resolve, we will not be divided and our resolve will hold firm. We will show by our spirit and dignity ... our values will long outlast theirs."
G-8 leaders condemned the attacks.
The U.N. Security Council also condemned the attacks in a unanimous resolution, drawn up by Britain, which expressed sympathy for the victims. It urged all nations to help bring those responsible to justice and expressed the council's determination to combat terrorism.
Much of Europe went on alert. Italy's airports raised their alert level to a maximum. The Czech Republic, Hungary, Russia, the Netherlands, France and Spain increased security at shopping centers and airports and on railways and subways.
The U.S. State Department set up a task force and call center for those with relatives and friends in London. The toll-free number is 1-888-407-4747.
Unknown Group Claims Responsibility
A previously unknown organization, the Secret Group of Al Qaeda's Jihad in Europe, claimed responsibility in the name of Al Qaeda for the blasts, saying they were in retaliation for Britain's involvement in Iraq and Afghanistan.
The group claimed the attack in a Web-site posting and also warned Italy and Denmark to withdraw troops from Iraq and Afghanistan.
Arabic satellite television station Al-Jazeera said it had received a phone call from the same group claiming responsibility.
Neither claim could be verified.
The Web site carrying the posting is popular with Islamic militants, according to Elaph, a secular Arabic-language news Web site, and Der Spiegel magazine in Berlin, which published the text on its German- and English-language Web sites.
"Rejoice, Islamic nation. Rejoice, Arab world. The time has come for vengeance against the Zionist crusader government of Britain in response to the massacres Britain committed in Iraq and Afghanistan," said the statement, as translated by The Associated Press in Cairo.
"The heroic mujahedeen carried out a blessed attack in London, and now Britain is burning with fear and terror, from north to south, east to west," the statement reads.
FBI and Justice Department officials said it was still too early to conclude that the London attacks were the work of Al Qaeda, but other experts were more certain.
"It has all the earmarks of Al Qaeda," Dennis Ross, special Middle East envoy for Presidents George H.W. Bush and Bill Clinton, told FOX News. "This has probably been planned for a long time. They probably had some sleeper cells in the U.K. for some time."
However, in previous Al Qaeda attacks, such as on Sept. 11, 2001, and the embassy bombings in Africa in 1998, there was no immediate claim of responsibility.
What was notable about Thursday's attacks was their extreme coordination, their timing during rush hour for maximum casualties and their targeting of the transit system, counterterrorism officials told FOX News.
The attacks came a day after London was awarded the 2012 Olympics and as the G-8 summit was getting underway in Scotland.
A spokesman for the Olympic committee said it still has "full confidence" in London as the host of the 2012 Games.
From Singapore, where he had been for the Olympic selection, Greater London Authority Mayor Ken Livingstone (search) said: "I want to say one thing, specifically to the world today — this was not a terrorist attack against the mighty and the powerful."
"It was not aimed at presidents or prime ministers," the former Trotskyist said. "It was aimed at ordinary, working-class Londoners, black and white, Muslim and Christian ... young and old … that isn't an ideology, it isn't even a perverted fate, it is an indiscriminate attempt at mass murder."
"They seek to divide London. ... This city of London is the greatest in the world because everybody lives side by side in harmony. Londoners will not be divided by this cowardly attack."
G-8 Leaders Stand United Against Terrorism
Blair, who was hosting the world's most powerful industrial leaders at Gleneagles, Scotland, left the G-8 summit to meet with police and other officials but said the rest of the leaders would remain. The G-8 gathering is focusing on climate change and aid for Africa — but Iraq has largely been left off the agenda.
"Each of the countries around that [G-8] table have experience with the effects of terrorism and all of the leaders ... share our complete resolution to defeat this terrorism," Blair said in his address Thursday.
"It's particularly barbaric this has happened on a day when people are meeting to try to help the problems of poverty in Africa, the long-term problems of climate change and the environment."
The G-8 leaders later issued a statement of their own. Blair, flanked by President George Bush on one side and French President Jacques Chirac on the other, read the statement on a stage accompanied by the other G-8 leaders in a sign of solidarity.
"Those responsible have no respect for human life. We are united in our resolve to confront and defeat this terrorism that is not an attack on one nation but on all nations and on civilized people everywhere. We will not allow violence to change our societies or our values, nor will we allow it to stop the work of this summit," the statement read.
"The terrorists will not succeed. Today's bombings will not weaken in any way our resolve to uphold the most deeply held principles of our societies and to defeat those who would impose their fanaticism and extremism on all of us. We shall prevail and they shall not."
Bush later told reporters that the people of London have America's "heartfelt condolences" and said he appreciates Blair's steadfast determination and strength.
"He'll carry a message of solidarity with him" as he leaves the G-8 summit for London, Bush added. "I was most impressed by the resolve of all the [G-8] leaders in the room and that their resolve is as strong as my resolve. ... We will not yield to the terrorists. We will find them; we will bring them to justice."
Bush said there's a clear contrast between the work being done at the G-8 summit and the goals of the terrorists responsible, "those who've got such evil in their heart that they will take the lives of innocent folks."
Blasts Hit Buses, Subway System
In London, police said they could confirm at least 37 people had been killed, including two in the bus attack. Three U.S. law enforcement officials told The Associated Press at least 40 were killed. French Interior Minister Nicolas Sarkozy later said the death toll had risen to 50, citing a conversation with his British counterpart, but that could not immediately be confirmed.
Police said at least 700 were wounded, many of whom emerged bleeding and dazed from the Underground. Among them, at least 45 were in serious or critical condition, including amputations, fractures and burns, hospital officials told The Associated Press.
Two young women from Knoxville, Tenn., were among those treated for injuries in the Underground, said their father, Dudley Benton.
The first blast caught a subway train between Moorgate and Liverpool Street stations, on the eastern fringe of London's financial district. Some people caught in the blast emerged from the Aldgate Station.
The second bombing came five minutes later, on a second train deep underground between the King's Cross and Russell Square stations. King's Cross station, in one of the seediest parts of London, is the film setting for Platform 9 3/4 in the Harry Potter films. Russell Square station serves Bloomsbury, the early 20th-century literary hotbed where Virginia Woolf and luminaries lived.
At 9:17 a.m., there was an explosion involving two or perhaps three trains around Edgware Road station. Edgware Road is the heart of a thriving Arab community, and convenient to Hyde Park, scene of last weekend's Live 8 concert.
The bus explosion took place near Russell Square, an area of many modestly priced hotels popular with tourists. Also nearby is the home where Charles Dickens lived from 1837 to 1839.
Bradley Anderson, a subway passenger, told Sky News that "there was some kind of explosion or something" as his train reached the Edgware Road station in northeast London.
"Everything went black and we collided into some kind of oncoming train," Anderson said.
Simon Corvett, 26, who was on an eastbound train from Edgware Road station, said: "All of a sudden there was this massive, huge bang."
"It was absolutely deafening and all the windows shattered," he said. "There were just loads of people screaming and the carriages filled with smoke. You could see the carriage opposite was completely gutted. There were some people in real trouble."
Jay Kumar, a business owner near the site of the blast that destroyed the bus, said he ran out of his shop when he heard a loud explosion. He said the top deck of the bus had collapsed, sending people tumbling to the floor.
Many appeared badly injured, and bloodied people ran from the scene.
"I was on the bus in front and heard an incredible bang. I turned round and half the double-decker bus was in the air," Belinda Seabrook told Press Association, the British news agency.
Home Secretary Charles Clarke (search), Britain's top law enforcement officer, said there were "terrible injuries."
Americans Urged to be Vigilant
U.S. officials said they had no intelligence that suggested similar attacks were planned for the United States; there are no plans currently to raise the terror alert system. But the U.S. rail system was placed on orange alert. Click here for that complete story.
"The United States government is raising the threat level from code yellow to orange, targeted only to the mass transit portion of the transportation sector," Homeland Security Secretary Michael Chertoff said in an afternoon press conference.
To the people of London, Chertoff said: "America stands with you in this time of crisis in attempts to help in any ways possible."
FOX News' Catherine Herridge, Scott Norvell, Anna Persky, Liza Porteus, Teri Schultz and The Associated Press contributed to this report.