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.