A top White House counterterrorism official tried to reassure the public Friday that it's safe to fly even as she acknowledged that investigators continue to search for some of the terrorists accused of planning to blow up airliners over the Atlantic Ocean.

Frances Fragos Townsend, President Bush's homeland security adviser, did not rule out that some missing plotters may be in the United States.

"There are leads that the FBI is running," Townsend told ABC's "Good Morning America" during a round of morning television interviews.

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"We are looking for connections between anyone in the United States and the plotters in the U.K., but we don't have any evidence that there is an active threat or cell here," she told CBS.

Though British officials have arrested two dozen alleged plotters, several suspects remain at large, including the suspected ring leaders of the London plot.

Hundreds of FBI agents tracked down leads around the United States in the past few weeks, but did not find any plotters in this country, FBI officials said.

The FBI is expecting the arrests and searches of homes and computers in England to generate another round of leads on possible U.S. ties. But there have been no arrests in the United States in connection with the plot, officials said.

Townsend said people should continue to fly.

"People ought to feel safe about traveling because of all the precautions we're taking," she said.

More will be learned about the plot in coming days, White House officials say. In the meantime, the U.S. airline system remains on high alert, with tougher passenger inspections expected Friday.

Details continued to emerge about the alleged plot, which officials said they had tracked for months. A 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.

British authorities arrested 24 people Thursday based partly on intelligence from Pakistan, where authorities detained up to three others days earlier. More arrests were expected, an official said.

For the U.S. traveling public, already heavy security restrictions got even worse. Thursday night, British Airways banned carry-on bags from all flights between the United States and Britain. On Friday, passengers in the U.S. will be subject to a second security check at airport gates to prevent anyone from carrying onto planes liquids that could be used in an explosion, airline officials said.

Bush said the foiled plot showed the nation was "at war with Islamic fascists who will use any means to destroy those of us who love freedom, to hurt our nation."

"This country is safer than it was prior to 9/11," Bush said Thursday in Green Bay, Wis. "We've taken a lot of measures to protect the American people, but obviously we're not completely safe. ... It is a mistake to believe there is no threat to the United States of America."

The evidence uncovered so far shows the plot aimed to cause unprecedented casualties.

"The terrorists intended a second Sept. 11," Townsend said on NBC's "Today."

Bush's spokesman earlier had declared "it is safe to travel." The president urged Americans to be patient with the added inconveniences that will result from the increased threat level that the plot prompted him to approve.

Authorities said terrorists were only days away from carrying out the plan to blow up as many as 10 airliners flying from Britain to the United States. They were about to try a test run to see whether innocent-looking explosive materials could be brought on board, U.S. officials say.

Homeland Security Secretary Michael Chertoff said the code red alert for Britain-to-U.S. flights doesn't mean "that we know for a fact there are people out there who are still active."

But he added that "particularly at this stage of the arrest and the takedown, there is sufficient uncertainty about whether the British have scooped up everybody."

He called the plot "a well-advanced plan" that was in "some respects suggestive of an Al Qaeda plot."

Two U.S. government officials, speaking on condition of anonymity because the information remains secret, said British, American and Pakistani investigators are trying to determine whether a couple of the suspects attended terrorist training camps in Pakistan.

Bush and British Prime Minister Tony Blair personally followed the developing drama before it became public, with discussions in a lengthy teleconference Sunday and a phone conversation Wednesday.

Intelligence officials watched the plot unfold until they could wait no longer because of the impending test run, officials said.

A red alert for flights from Britain was the first since the color-coded warning system was developed after the Sept. 11, 2001, terror attacks. The decision to ban nearly all liquids from passenger cabins was reminiscent of the stringent rules imposed when planes returned to the skies after the 2001 attacks.

All other flights to and within the United States were put under an orange, or high, alert Thursday, one step below red but up from the yellow status that had been in effect.

Chertoff said no liquids or gels will be allowed in carry-on bags, except for baby formula and medicines. But travelers must be prepared to present those items for inspection at checkpoints "and that will allow us to take a look at them and make sure that they're safe."

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.

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 said. The officials spoke on condition of anonymity because Britain asked that no information be released.