The terror scheme disrupted in London is "suggestive of an al-Qaida plot," the Bush administration said Thursday as it issued its highest terrorism alert ever for commercial flights from Britain to the United States and raised the threat level for all domestic and international flights.
Homeland Security Secretary Michael Chertoff said there was no indication of plotting in the United States but said officials cannot assume that the terror operation in Britain had been completely thwarted.
The administration raised the threat level for flights from Britain to "red," designating a severe risk of terrorist attacks. All other flights, including all domestic flights in the United States, were put under an "orange," alert - one step below the highest level.
The U.S. government banned all liquids and gels from flights, including toothpaste, makeup, suntan lotion. Baby formula and medicines were exempted.
Chertoff said the alleged plot appeared to be engineered by al-Qaida, the terrorist group that carried out the Sept. 11, 2001, attack against the United States.
"It was sophisticated, it had a lot of members and it was international in scope," said Chertoff. "It was in some respects suggestive of an al-Qaida plot."
He added, however, that "because the investigation is still underway we cannot yet form a definitive conclusion."
Chertoff said the plotters were in the final stages of planning before execution. "We were really getting quite close to the execution phase," he said. He said it was unclear whether the alleged plot was linked to the upcoming fifth anniversary of the Sept. 11 strikes.
Attorney General Alberto Gonzales said the operation could "potentially kill hundreds of innocent people." Britain said 21 people had been arrested, including the alleged "main players" in the plot.
FBI Director Robert Mueller also pointed at al-Qaida. "This had the earmarks of an al-Qaida plot," he said.
Chertoff said it envisioned multiple explosions in multiple aircraft.
The new security measures caused long backups and delays at airport security checkpoints throughout the United States and around the world. "We are taking some very serious and inconvenient measures," Chertoff said. He said it was advisable to have more protection and scale it back.
The alleged plot was "as sophisticated as any we have seen in recent years as far as terrorism is concerned," Chertoff said.
Chertoff said there was no indication of any plotting in the United States but the United States was taking step to protect against unseen threats or copycat attacks. "We cannot assume that this threat has been completely thwarted," the secretary said.
"There's sufficient uncertainty as to whether the British have scooped up everybody," Chertoff said.
British authorities said the plan envisioned bringing down a number of aircraft with midflight explosions. Chertoff said the terrorists planned to bring various bomb components in a benign state aboard the planes and combine them once the planes were aloft to create and detonate explosive devices.
Terrorists specifically had targeted United, American and Continental airlines, two U.S. counterterrorism officials said. One said the terrorists had hoped to target flights to major airports in New York, Washington and California, all major summer tourist destinations.
American and United flights were turned into terrorist weapons on Sept. 11, 2001, when they were hijacked and crashed.
Hastily printed signs were posted at major airports warning passengers in red capital letters, "No liquid or gels permitted beyond security."
It is the first time the red alert level in the Homeland Security warning system has been invoked, although there have been brief periods in the past when the orange level was applied. Homeland Security defines the red alert as designating a "severe risk of terrorist attacks."
Officials said the government has been aware of the nature of the threat for several days, and President Bush was fully briefed.
Initial word of the increased U.S. threat level came in an overnight written statement issued by Chertoff.
The plot was not believed to be connected to the Egyptian students who disappeared in the United States more than a week ago before reaching a college they were supposed to attend in Montana. Three of the 11 have since been found and the FBI has said neither they nor the still-missing eight are believed to be a threat.
As part of the foiled Bojinka Plot to blow up 12 Western airliners simultaneously over the Pacific Ocean in the mid-1990s, terrorist mastermind Ramzi Youssef planned to put together an improvised bomb using liquid in a contact lens solution container.
The metal detector and X-ray machines at airport security checkpoints cannot detect such explosives. At many, but not all airport checkpoints, the TSA has deployed walkthrough "sniffer" or "puffer" machines that can detect explosives residue.
Prime Minister Tony Blair's office said in London that the prime minister, vacationing in the Caribbean, had briefed Bush on the situation overnight.
The Homeland Security Department devised the alert system after the Sept. 11 attacks. The last time the U.S. government raised the terrorist risk here to orange, or high, was in July 2005 after the subway bombings in London. It was lowered to yellow a month later, the elevated risk status that has been the norm since the system was created.
American Airlines parent is AMR Corp.
United's parent is UAL Corp.
Continental's full name is: Continental Airlines Inc.
Associated Press writers Katherine Shrader, Mark Sherman, Leslie Miller and Robert Burns contributed to this report.