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Associated Press
An armed police officer stands guard outside Terminal 4 at London's Heathrow Airport. Britain is still seeking five suspects in a plot to blow up as many as 10 airplanes flying to the United States. The attack was thought to be imminent.

LONDON — With other suspects in an alleged terrorist plot to blow up U.S.-bound jetliners still at large late Thursday, officials weren't saying how long airport security measures employed for imminent terrorist threats will stay in place here and in the United States.

"We want to make sure that there are no remaining threats out there, and we also want to take steps to prevent any would-be copycats who may be inspired to similar conduct," said U.S. Homeland Security Secretary Michael Chertoff.

Terror threat levels were raised to the top of their scales and hundreds of flights were canceled worldwide Thursday after British police said they thwarted a terrorist plot, possibly just days away, that would have killed thousands of travelers aboard as many as 10 flights.

"Quite simply, this was intended to be mass murder on an unimaginable scale," said Paul Stephenson, deputy chief of London's Metropolitan Police.

Chilling accounts leaked by investigators described a plan on the scale of 9/11 that would use common electronic devices to detonate liquid explosives concealed in sports drink bottles to bring down as many as 10 planes in a nearly simultaneous strike.

The bombs were to be assembled on the aircraft apparently with peroxide-based solution and everyday carry-on items such as a disposable camera or a music player, two American law enforcement officials told The Associated Press. The officials spoke on condition of anonymity because Britain asked that no information be released.

A federal law enforcement official in Washington said that at least one martyrdom tape was found during ongoing raids across England on Thursday. Such a tape, as well as the scheme to strike a range of targets at roughly the same time, is an earmark of al-Qaida.

British authorities arrested 24 people based partly on intelligence from Pakistan, where authorities detained up to three British nationals several days earlier. The suspects were believed to be mainly British Muslims, at least some of them of Pakistani ancestry.

The Bank of England froze the assets of 19 people early today 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. Intelligence officials said they believed some plotters were probably still at large.

A senior U.S. counterterrorism official said the suspects were looking to sneak at least some chemicals on the planes in sports drink bottles. Teams of at least two or three men were assigned to each flight, the schedules for which they had researched on the Internet, the official said.

A British anti-terror official, who like others spoke on condition of anonymity because the investigation is ongoing, said the suspects planned to blow up as many as 10 U.S.-bound planes in waves of three over the mid-Atlantic. They had researched flight routes and determined that U.S.-bound jets tend to fly in batches toward their destinations, the official said.

"The planes would simply disappear and it would be impossible to recover forensic evidence needed for investigation," the official said.

Two other U.S. officials said British, American and Pakistani investigators were trying to trace the steps of the suspects in Pakistan and were seeking to determine whether a couple of them attended terrorist training camps there.

American investigators praised Britain for preventing a catastrophe. "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.

With heightened security at airports, passengers stood in line for hours and airport trash bins bulged with everything from mouthwash and shaving cream to maple syrup and fine wine. Governors in at least three U.S. states — California, New York and Massachusetts — ordered National Guard troops to help provide security.

Experts 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.

Although plots to blow up airliners using liquid explosives are not new — such an attempt was foiled more than a decade ago — the U.S. government has been slow to upgrade its security equipment at airport checkpoints to detect explosives on passengers.

Experts said airliners are traditional targets for terrorists because they symbolize a nation's policy and tend to be easy targets that can wreak economic havoc.

"Terrorists always go after the weak link," said Charles Pena, a senior fellow at George Washington University's homeland security institute. "They saw this potential weak link in the aviation system. They're going to continue to look for potential weaknesses in aircraft security, but not to the exclusion of other targets they might be interested in attacking."

The raids in Britain on Thursday followed a monthslong investigation, but U.S. intelligence officials said authorities moved quickly after learning the plotters hoped to stage a "dry run" within two days, with the actual attack expected just days after that.

The test run was designed to see whether the plotters would be able to smuggle the needed materials aboard the planes, the officials said, speaking on condition of anonymity.

Targeted were United, American and Continental Airlines flights from Britain to major U.S. destinations, which counterterrorism officials said probably included New York, Los Angeles and Washington, D.C. Los Angeles Mayor Antonio Villaraigosa said the individuals plotted to detonate liquid explosive devices on as many as 10 aircraft.

President Bush, who has been on a working vacation on his Texas ranch near Crawford, had known about the investigation for at least several days. He received "full briefings" about the alleged plot over the weekend and had two conversations about the imminent threat with British Prime Minister Tony Blair, said White House press secretary Tony Snow.

"This nation is at war with Islamic fascists who will use any means to destroy those of us who love freedom, to hurt our nation," Bush declared.

Bush acknowledged airport restrictions would annoy travelers and urged patience.

The plane bombings could have come just ahead of the fifth anniversary of the Sept. 11 attacks carried out by al-Qaida. The terror group's leader Osama bin Laden and his No. 2, Ayman al-Zawahri, are believed hiding along the Pakistan-Afghanistan border and have repeatedly issued tapes threatening new attacks.

The close call also shifted attention once more to Britain's Islamic community just over a year after the London transit attacks. 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.

French Interior Minister Nicholas Sarkozy said the group "appears to be of Pakistani origin," but did not give a precise source for the information. Britain's Home Office refused comment.

A British police official, speaking on condition of anonymity because of the sensitivity of the investigation, said the suspects were "homegrown," though it was not immediately clear if all were British citizens. He said authorities were working with Britain's large South Asian community.

Tariq Azim Khan, the Pakistani minister of state for information, said "these people were born and brought up in the United Kingdom. Some of them may have parents who were immigrants from Pakistan."

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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.

Hamza Ghafoor, 20, who lives across the street from one of the homes raided in Walthamstow, northeast of London, said police circled the block in vans Wednesday and that they generally swoop into the neighborhood to question "anyone with a beard."

"Ibrahim didn't do nothing wrong," Ghafoor said, referring to a suspect. "He played football. He goes to the mosque. He's a nice guy."