America may have been just days away from another Sept. 11, Department of Homeland Security officials said.
That dire assessment came hours after British police swooped in and arrested 24 people in London Thursday, busting up an alleged terrorist plot to blow 10 packed U.S.-bound jetliners out of the sky over the Atlantic.
The plot was foiled when an undercover British agent infiltrated the UK-based group, and passed information to authorities, FOX News has learned.
Early Friday, the names of 19 of the 24 suspects accused in the plotting were made public by British officials.
And it became clear information that played "a very important role" in the subsequent arrest of the 24 in London was provided to Pakistani officials by British nationals arrested in Pakistan in connection with the plot a week ago, according to a government official there. (Full story)
Five Pakistanis were arrested Friday on suspicion that they served as local "facilitators" for those two Britons, the official said.
British Home Secretary John Reid said his country was grateful for Pakistan's cooperation.
The suspects in the plot allegedly planned to smuggle peroxide-based liquid ingredients disguised as sports drinks that could be mixed aboard the flight to produce an explosive compound possibly detonated by a device disguised as a digital camera or audio player, Department of Homeland Security Director Michael Chertoff to FOX News' Bill O'Reilly.
"I think we were facing the possibility of someone bringing in individual components, each of which would look very innocuous and very common, and literally assembling the bomb in the final minutes on the plane itself," he said.
U.S. counterterrorism officials said the suspects targeted United, American, and Continental airlines flights bound for New York, Washington and California.
The Bank of England froze the assets of 19 people early Friday and released their names, saying they had been arrested on Thursday. All had Muslim names, many of which are common in Pakistan. The youngest person was 17, the oldest 35. (Full story)
Meanwhile, the DHS raised the terror alert level for commercial flights from Britain to the U.S. to code-red. It's the first time the U.S. has raised the alert level to its maximum since the system was instituted in March 2002. Chertoff said the decision to raise the alert level was made after British officials bumped their terror alert level to their highest.
Reid said Friday the British threat level would remain at "critical."
Passengers endured long lines and arduous waits as airport security maticulously searched baggage for liquids and gels, which, except for baby food, were barred from all flights in Britain and the U.S.
Airport trash bins bulged with everything from mouthwash and shaving cream to maple syrup and fine wine. Click here for full story on new carry-on restrictions and procedures.
There was no word on how long the heightened security measures would last, but Chertoff said that once authorities are able to get their hands on the devices, reverse engineers may be able analyze them and then recalibrate detection equipment to identify such devices in the future.
"While we are doing that, we are going to be safe rather than sorry ... and that's why we are putting this measure in effect about screening all liquids out," Chertoff said.
A senior U.S. counterterrorism official said authorities believe dozens of people — possibly as many as 50 — were involved in the plot, and that more arrests were expected. The official spoke on condition of anonymity because of the sensitivity of the situation.
"We think the British succeeded in disrupting the plot ... but their judgment was that, given the haste in which they had to move, they could not be a hundred [percent] sure that they scooped up everybody nor were they sure that there weren't other affiliated parts of the network still at large and still capable of carrying out a bombing," Chertoff said.
Tariq Azim Khan, the Pakistani minister of state for information, said about the suspects that "these people were born and brought up in the United Kingdom. Some of them may have parents who were immigrants from Pakistan."
The raids were based partly on intelligence emanating from Pakistan where authorities detained three other suspects several days ago. British and U.S. intelligence officials decided to move quickly after learning the plotters planned to stage a dry-run within two days, with the actual attack expected just days after that, U.S. intelligence officers said on condition of anonymity.
Raids were carried out at homes in London, the nearby town of High Wycombe and in Birmingham, in central England. Searches continued throughout the day, and police cordoned off streets in several locations. Police also combed a wooded area in High Wycombe. British authorities said they were "urgently" seeking the arrests of up to 10 more suspects.
At least one martyrdom tape was found, a federal law enforcement official in Washington said. Such tapes, as well as the scheme to strike a range of targets at roughly the same time, are hallmarks of Al Qaeda plots.
A U.S. congressman briefed by intelligence officials, who did not want to be identified because of the sensitivity of the investigation, said U.S. intelligence had intercepted terrorist chatter and British intelligence helped thwart the plot through undercover work.
"If this plot had actually occurred, the world would have stood still," Mark Mershon, assistant director of the FBI, told the AP in New York.
Although American, British and Pakistani authorities repeatedly referred to Al Qaeda, all of them stop short of directly accusing Usama bin Laden 's terror network. The plane bombings could have come just ahead of the fifth anniversary of Sept. 11.
"In terms of scale, it was probably designed to be ... a new Sept. 11," said Jean-Charles Brisard, a French private investigator who works with lawyers of many Sept. 11 victims. "It involved the same tools, the same transportation tools and devices."
President Bush called the plot a "stark reminder" of the continued threat to the United States from extremist Muslims. (Full Story)
"It is a mistake to believe that there is no threat to the United States of America, and that is why we have given our officials the tools they need to keep Americans safe," Bush said.
Plots to blow up airliners using liquid explosives are not new. In 1995, officials foiled a plan by terrorist mastermind Ramzi Youssef, whose uncle Khalid Shaikh Mohammed oversaw the Sept. 11 attacks, to blow up 12 Western jetliners simultaneously over the Pacific. The alleged plot involved improvised bombs using liquid hidden in contact lens solution containers. Yousef is now an inmate at the Supermax federal prison in Florence, Colorado. (Full Story)
Experts, however, said the nature of the plot could herald a new age of terrorism where attackers have access to explosives that are easy to carry and conceal. Emergency security measures quickly implemented on Thursday provided a stark vision of the possible future of air travel.
Mothers tasted baby food in front of airport security guards to prove it contained no liquid explosives. Liquids and gels were banned from flights. Travelers repacked their luggage in airports, stowing all but the most necessary items in the hold.
The close call also shifted attention once more to Britain's Islamic community just over a year after the July 7 London transit attacks that killed 52. Three Britons of Pakistani descent and a Jamaican convert to Islam carried out those deadly bombings with a peroxide-based explosive that trained operatives can make using ordinary ingredients such as hair bleach.
Homeland Security Deputy Secretary Michael Jackson said his agency had known for several days of the unfolding plot but waited for a signal from the British to announce it.
All other flights to and within the United States were put under an orange alert, one step below red, but an escalation from the yellow status that had been in effect.
Administration officials sought to reassure the traveling public at the same time they imposed heightened security restrictions.
"Today, air traffic is safe, and air traffic will remain safe precisely because of the measures we are adopting today," Chertoff said.
FOXNews.com's C. Spencer Beggs and The Associated Press contributed to this report.