With two subway lines closed Friday morning and the twisted wreckage of a double-decker bus covered by tarps, London (search) authorities searched for answers and suspects in the coordinated series of attacks that killed at least 50 people.

And they're asking anyone with information about the attacks to come forward.

"Together we can defeat this. The communities defeat terrorism, not the police," Metropolitan Police Commissioner Sir Ian Blair said during an afternoon press conference Friday. "We remain … full of implacable resolve to track down the people responsible for this atrocity."

Officials increased the death toll from Thursday's attacks Friday afternoon when they announced that 13 people had died in the attack on a double-decker bus; all bodies have been recovered from the bus, they said. The death toll could increase, however, since not all bodies have been pulled from one targeted underground car.

Police said investigation and recovery were hampered at a bomb site because of fears the subway tunnel was unsafe. Engineers were examining structural damage.

Blair confirmed during an earlier news conference Friday that more than 700 people were wounded in the attacks. One hundred were hospitalized overnight, 22 of them in critical condition. Plus, he indicated that authorities had not yet determined the violence was caused by a homicide bomber or remotely-detonated bombs.

"There is absolutely nothing to suggest that these were suicide attacks," Blair said, though he was careful to note that the possibility had not been ruled out, either.

London Mayor Ken Livingstone also encouraged Londoners to report anything they remember seeing or hearing that seemed out of place the day of, or leading up to, the attacks.

"Don't feel shy, come forward, let us have that information now. It may be decisive in catching those people who did these terrible crimes," Livingstone said.

Officials at Scotland Yard (search) said initial investigation showed that each of the detonated devices apparently consisted of no more than 10 pounds of explosives, and that there was no evidence bombers were aboard the trains or the bus at the time of the blasts. It's believed the bombs were placed on the floors of the three subway cars that were hit.

Police officials also stressed Friday that there were only four explosions; three on the subway and one on a double-decker bus during the morning rush hour. There was confusion Thursday over the exact number due to train passengers fleeing single attack sites from more than one station.

Assistant Police Commissioner Andy Hayman appealed for patience as the investigation proceeds. "Our people are working under the most extreme circumstances."

Investigators are reviewing the video footage from the 1,800 or so cameras in London's train stations.

Prime Minister Tony Blair (search) appeared with leaders of the world's eight major industrialized nations, as well as leaders of African countries, on Friday again to address the globe. Speaking from the G-8 (search) summit in Gleneagles, Scotland, Blair emphasizing that terrorism will not derail the work being done there to reduce poverty, eliminate the AIDS pandemic and clean up the environment.

"We speak today in the shadow of terrorism but it will not obscure what we came here to achieve," Blair said. "There is no hope in terrorism nor any future in it worth living and it is hope that is the alternative to this hatred. So we offer today this contrast with the politics of terror."

For more, go to these FOX News partners: Sky News | The Times of London | The Sun

Was Al Qaeda Behind the Attacks?

Much of the focus Friday turned to the hunt for who was responsible.

"It is simply a criminal attempt at mass murder," Livingstone said.

An organization calling itself the "Secret Group of Al Qaeda's Jihad in Europe (search)" claimed responsibility for the attacks; the group said the blasts were in retaliation for Britain's involvement in Iraq and Afghanistan. It threatened to attack Italy and Denmark for their support of the U.S.-led coalitions in both countries, too. That claim couldn't immediately be confirmed.

Charles Shoebridge, a security analyst and former counterterrorism intelligence officer, said detectives will watch thousands of hours of video to try to find the point at which bombs were placed, then trace back the movements of the bomber. Investigators also will check records of cell-phone calls made in the bombed areas just before the explosions.

"There is real passion now in the police to make arrests quickly before further attacks can be carried out," he said.

Investigators believe some of the bombs were on timers, a U.S. law enforcement official said, but they doubt that cell phones — used in the Madrid train attacks a year ago — were used to detonate the bombs because the phones often don't work in the system's tunnels, the official said. Police denied an earlier claim by a U.S. official who said unexploded devices had been found.

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.

Britain's prime minister blamed Islamic extremists and said the bombings were designed to coincide with the opening in Scotland of a G-8 summit of the world's most powerful leaders.

"It's not because of something we're doing wrong that these barbaric terrorists attack us, it's because of what we're doing right," New York Gov. George Pataki told FOX News on Friday. "They'll always detest those freedoms, we have always got to defend those freedoms."

Foreign Secretary Jack Straw said the bombings — which came the day after London won the bid to host the 2012 Olympics (search) — have the "hallmarks of an Al Qaeda-related attack."

Terrorism experts agreed that the explosions had the hallmarks of Al Qaeda (search).

"This is clearly an Al Qaeda style attack. It was well coordinated, it was timed for a political event and it was a multiple attack on a transportation system at rush hour," said Lawrence Freedman, professor of war studies at King's College in London.

'We'll Carry On'

Tube stations opened around 5:30 a.m. Friday, their normal time, although delays were expected due to partially or fully closed subway routes. Buses in London were to resume their usual routes with no foreseen closures.

Of the residents who reluctantly ventured back to the partially reopened Underground, some said they had little choice but to return to mass transit.

"I was scared, but what can you do?" said Raj Varatharaj, 32, emerging from an Underground station. "This is the fastest way for me to get to work. You just have to carry on."

"As Brits, we'll carry on — it doesn't scare us at all," said tour guide Michael Cahill, 37. "Look, loads of people are walking down the streets. It's Great Britain - not called 'Great' for nothing."

Shows across London's West End theater district were canceled Thursday while shops and banks in the capital closed their doors early.

Weekend concerts by Queen and REM in Hyde Park also were postponed by a week because of safety concerns.

Stocks were higher in Europe on Friday, with insurance and travel-related stocks regaining some of the ground they lost on Thursday.

Queen Elizabeth II, her son Prince Charles and his wife, Camilla, Duchess of Cornwall, meanwhile, made separate visits to bombing victims at hospitals in the capital.

The queen expressed her admiration for all the Londoners who "are calmly determined to resume their normal lives."

"Sadly we in Britain have been all too familiar with acts of terror and members of my generation, especially at this end of London, know that we have been here before," she said during a visit to the Royal London Hospital, referring to the Nazi air blitz of World War II.

"But those who perpetrate these brutal acts against innocent people should know that they will not change our way of life," she stressed.

Of the subway system's 12 lines, service on two — the Circle Line and the Hammersmith and City Line — was suspended.

Euston Station was briefly evacuated after a suspicious package was found. Bus service was running through central London, except for diversions around blast sites.

Authorities on Thursday 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.

Police said there had been no warning and that 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.

"There is nothing to suggest that intelligence has been missed in any way," Commissioner Blair said.

Security was raised in the United States and around the world. The Bush administration upped the terror alert a notch to code orange for the nation's mass transit systems, and bomb-sniffing dogs and armed police patrolled subways and buses in the capital.

Homeland Security Secretary Michael Chertoff (search) told FOX News on Friday that the security level was raised over concerns of a copycat attack, not because of particular information.

"There has not been specific intelligence concerning a particular attack on the rail system," Chertoff said.

Much of Europe also went on alert, and Italy's airports raised alert levels to a maximum.

FOX News' Eric Margolis, Alex Duncan and The Associated Press contributed to this report.

For more, go to these FOX News partners: Sky News | The Times of London | The Sun