British police allegedly found the tape during raids that netted 24 people. More arrests are expected.
The terrorist attack foiled by British authorities on Thursday was aimed at blowing up as many as 10 airplanes on trans-Atlantic flights, and plotters had hoped to stage a dry run within two days, U.S. intelligence officials said. The actual attack, they said, was expected just days after that.
The officials said the test run was designed to see whether the plotters would be able to smuggle the needed materials aboard the planes. They spoke only on condition of anonymity because of the sensitivity of the subject matter.
American law enforcement sources said terrorists planned to use a peroxide-based explosive that would have been detonated by power from a camera, a digital music player or other electronic device.
The plane bombings could have come just ahead of the fifth anniversary of the Sept. 11, 2001, terrorist attacks carried out by al-Qaida.
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.