Suspects arrested Thursday for planning to stage a massive mid-air terror attacked were in the final stages of planning and planned to run a dry-run of the plan within two days, U.S. intelligence officers said Thursday.
One official said the suicide attackers planned to use a peroxide-based solution that could ignite when sparked by a camera flash or another electronic device.
The test run was designed to see whether the plotters would be able to smuggle the needed materials aboard the planes, these officials said. They spoke only on condition of anonymity because of the sensitivity of the subject matter.
The development came as British authorities said they were "urgently" seeking the arrests of up to 10 more suspects in the terrorist plot uncovered early Thursday morning to blow up U.S.-bound flights with liquid explosives carried onto planes via carry-on luggage, FOX News learned.
Police arrested 24 main suspects were arrested earlier Thursday, according to Scotland Yard, in what U.S. officials suspect was an Al Qaeda-planned attack.
French Interior Minister Nicolas Sarkozy said the suspects in an alleged plot to "appear to be of Pakistani origin," and Pakistani intelligence officials say they assisted in the arrests.
Pakistani intelligence officials helped British security agencies crack a terror plot to blow up U.S.-bound aircraft from Britain and arrested two or three suspects in recent days, authorities said Thursday. (Full story)
The intelligence official said an Islamic militant arrested near the Afghan-Pakistan border several weeks ago provided a lead that played a role in "unearthing the plot," that helped authorities arrest suspects in Britain.
President Bush called the plot a "stark reminder" of the continued threat to the United States from extremist Muslims. (Full story)
Britain disclosed no details about the plot or those arrested, although one police official indicated the people in custody were British residents. A French official in contact with British authorities described the arrested as originating from predominantly Muslim Pakistan.
Officials raised security to its highest level in Britain — suggesting a terrorist attack might be imminent — and banned carry-on luggage on all flights. Huge crowds backed up at security barriers at London's Heathrow airport as officials searching for explosives barred nearly every form of liquid outside of baby formula.
U.S. Homeland Security Secretary Michael Chertoff said the terrorists planned to use liquid explosives disguised as beverages and other common products and set them off with detonators disguised as electronic devices.
An American law enforcement official who was briefed on the investigation said it appeared the liquid to be used was a "peroxide-based solution" to be detonated by an electronic device that was not specified, but could be anything from a disposable camera to a portable digital music player. The official spoke on condition of anonymity because British authorities had asked that no information be released.
The extreme measures at a major international aviation hub sent ripples throughout the world. Heathrow was closed to most flights from Europe, and British Airways canceled all its flights between the airport and points in Britain, Europe and Libya. Numerous flights from U.S. cities to Britain were canceled.
Washington raised its threat alert to its highest level for commercial flights from Britain to the United States amid fears the plot had not been completely crushed. The alert for all flights coming or going from the United States was also raised slightly.
Two U.S. counterterrorism officials said the terrorists had targeted United, American and Continental airlines. They spoke on condition of anonymity because of the sensitivity of the case.
A U.S. intelligence official said the plotters had hoped to target flights to major airports in New York, Washington and California. A counterterrorism official said the plot involved 10 flights.
British authorities said 24 people were arrested in London, its suburbs and Birmingham following a lengthy investigation, including the alleged "main players" in the plot. Searches continued in a number of locations, and police cordoned off streets in several locations.
Bush said during a visit to Green Bay, Wis., that the foiled plot was a "stark reminder that this nation is at war with Islamic fascists." Despite increased security since Sept. 11, he warned, "It is a mistake to believe there is no threat to the United States of America."
While British officials declined to publicly identify the 24 suspects, French Interior Minister Nicolas Sarkozy said in Paris they "appear to be of Pakistani origin." He did not give a source for his description, but said French officials had been in close contact with British authorities.
Pakistan's government said later its intelligence agents helped Britain crack the plot and had arrested some suspects.
"Pakistan played a very important role in uncovering and breaking this international terrorist network," Foreign Ministry spokesman Tasnim Aslam said, but she declined to give details.
The suspects arrested in Britain were "homegrown," though it was not immediately clear if they were all British citizens, said a British police official who spoke on condition of anonymity because of the sensitivity of the case. Police were working closely with the South Asian community, the official said.
The suicide bombing assault on London subway trains and a bus on July 7, 2005, was carried out by Muslim extremists who grew up in Britain.
The police official said the plotters intended to simultaneously target multiple planes bound for the United States.
"We think this was an extraordinarily serious plot and we are confident that we've prevented an attempt to commit mass murder on an unimaginable scale," Deputy Police Commissioner Paul Stephenson said.
Prime Minister Tony Blair, vacationing in the Caribbean, briefed Bush on the situation Wednesday. Blair issued a statement praising the cooperation between the two countries, saying it "underlines the threat we face and our determination to counter it."
White House spokesman Tony Snow said Bush also had been briefed by his aides while at his ranch in Crawford, Texas, where he has been on vacation.
"We do believe the plot involved flights from the U.K. to the U.S. and was a direct threat to the United States," Snow said.
While Snow called the plot a serious threat, he assured Americans that "it is safe to travel."
Chertoff, the homeland security chief, said the plot had the hallmarks of an operation planned by al-Qaida, the terrorist group behind the Sept. 11 attack on the United States.
"It was sophisticated, it had a lot of members and it was international in scope. It was in some respects suggestive of an al-Qaida plot," Chertoff said, but he cautioned it was too early in the investigation to reach any conclusions.
It is the first time the red alert level in the Homeland Security warning system has been invoked, although there have been brief periods in the past when the orange level was applied. Homeland Security defines the red alert as designating a "severe risk of terrorist attacks."
"We believe that these arrests (in London) have significantly disrupted the threat, but we cannot be sure that the threat has been entirely eliminated or the plot completely thwarted," Chertoff said.
He added, however, there was no indication of current plots within the United States.
Chertoff said the plotters were in the final stages of planning. "We were really getting quite close to the execution phase," he said, adding that it was unclear if the plot was linked to the upcoming fifth anniversary of the Sept. 11 terror attacks.
A senior U.S. counterterrorism official said authorities believe dozens of people — possibly as many as 50 — were involved in the plot. The official spoke on condition of anonymity because of the sensitivity of the situation.
The plan involved airline passengers hiding masked explosives in carry-on luggage, the official said. "They were not yet sitting on an airplane," but were very close to traveling, the official said, calling the plot "the real deal."
Passengers in Britain faced delays as tighter security was hastily enforced at the country's airports and additional measures were put in place for all flights. Laptop computers, mobile phones, digital music players, and remote controls were among the items banned from being carried on board.
Liquids, such as hair care products, were also barred on flights in both Britain and the U.S.
In the mid-1990s, officials foiled a plan by terrorist mastermind Ramzi Youssef to blow up 12 Western jetliners simultaneously over the Pacific. The alleged plot involved improvised bombs using liquid hidden in contact lens solution containers.
Huge lines formed at ticket counters and behind security barriers at Heathrow and other airports in Britain.
Ed Lappen, 55, a businessman from Boston, who was traveling with his wife and daughter to Russia, found himself unable to travel further. "We're safe, we're OK," he said at Heathrow. "Now my daughter is going to get a shopping trip in London."
Hannah Pillinger, 24, seemed less concerned by the announcement. "Eight hours without an iPod, that's the most inconvenient thing," she said, waiting at the Manchester airport.
Most European carriers canceled flights to Heathrow because of the massive delays created after authorities enforced strict new regulations banning most hand baggage.
Tony Douglas, Heathrow's managing director, said the airport hoped to resume normal operations Friday, but passengers would still face delays and a ban on cabin baggage "for the foreseeable future."
Security also was stepped up at train stations serving airports across Britain, said British Transport Police spokeswoman Jan O'Neill. At London's Victoria Station, police patrolled platforms with bomb-sniffing dogs as passengers boarded trains carrying clear plastic bags.
Margaret Gavin, 67, waiting to board a train, said she wasn't scared. "Why should I change my life because some idiots want to blow something up?" she said.
The Associated Press contributed to this report.