(AP) - 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 details of the alleged plot surfaced as the administration posted a maximum code-red alert for passenger flights from England to the United States and banned liquids from all carry-on bags.
The security upgrade triggered long lines at airports across the country, and governors in at least two states activated National Guard troops to help provide protection.
"This was a well-advanced plan," Homeland Security Secretary Michael Chertoff told reporters as British authorities announced the arrests of 24 alleged plotters. "In some respects suggestive of an al-Qaida plot."
Los Angeles Mayor Antonio Villaraigosa said as many as 10 flights had been targeted.
Other officials said they were United, American and Continental Airlines routes from Britain to the major U.S. summer tourist destinations of New York, California and Washington, D.C. These officials declined to provide details on when the plotters intended to strike.
Virginia's deputy homeland security director, Steven Mondul, said that in a morning conference call, federal officials pointed to New York's John F. Kennedy Airport, Los Angeles International and Dulles Airport outside Washington as "major destinations for flights originating from the United Kingdom." No specific warnings were issued for these facilities, he added.
The red alert for flights from Britain was the first since the color-coded warning system was developed in the wake of the 2001 terror attacks.
The decision to ban nearly all liquids from passenger cabins was reminiscent of the stringent rules imposed when planes were allowed back in the skies for the first time afterward the Sept. 11 attacks.
"No liquids or gels will be allowed in carry-on baggage," Chertoff said. "There will be exceptions for baby formula and medicines, but travelers must be prepared to present these items for inspection at the checkpoint, and that will allow us to take a look at them and make sure that they're safe to fly."
That meant water containers, soft drinks, coffee cups and more had to be shed by passengers waiting to board their flights.
Women travelers surrendered bottles and jars of creams and lotions from their makeup kits.
At Dulles, one passenger fished a bottle of Tequila from a carry-on bag. It joined the rest of the newly classified contraband in a trash container.
The decision to raise the terror level for flights from Britain to "red" indicated a severe risk of terror attacks. The change requires airlines to provide the government with an advance list of passengers aboard affected flights. Previously, passengers names had to be provided within 15 minutes after take-off.
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.
White House spokesman Tony Snow said President Bush had been briefed in advance of the events, and had approved raising the alert to red on flights from England.
In brief remarks from Green Bay, Wis., the president said the events showed the nation "is at war with Islamic fascists."
Senior lawmakers also received advance word. Several said they had been briefed by Homeland Security or CIA officials as early as Monday.
Word of the plot quickly became grist for the midterm election campaign, with less than 100 days to run.
House Democratic Leader Nancy Pelosi said events reinforced the need to implement the recommendations of an independent 9/11 commission, a reminder of one of her party's main campaign promises.
In Ohio, Republican chairman Bob Bennett accused the Democrats' senatorial challenger of voting against funds "for the very types of programs that helped the British thwart these vicious attacks."
(Copyright 2006 by The Associated Press. All Rights Reserved.)