A British anti-terror official, who like others spoke on condition of anonymity because the investigation is ongoing, said the suspects planned to blow up as many as 10 U.S.-bound planes in waves of three over the mid-Atlantic. They had researched flight routes and determined that U.S.-bound jets tend to fly in batches toward their destinations, the official said.

"The planes would simply disappear and it would be impossible to recover forensic evidence needed for investigation," the official said.

Two other U.S. officials said British, American and Pakistani investigators were trying to trace the steps of the suspects in Pakistan and were seeking to determine whether a couple of them attended terrorist training camps there.

American investigators praised Britain for preventing a catastrophe. "If this plot had actually occurred, the world would have stood still," Mark Mershon, assistant director of the FBI, told the AP in New York.

With heightened security at airports, passengers stood in line for hours and airport trash bins bulged with everything from mouthwash and shaving cream to maple syrup and fine wine. Governors in at least three U.S. states — California, New York and Massachusetts — ordered National Guard troops to help provide security.

Experts said the nature of the plot could herald a new age of terrorism where attackers have access to explosives that are easy to carry and conceal. Emergency security measures quickly implemented on Thursday provided a stark vision of the possible future of air travel.

Mothers tasted baby food in front of airport security guards to prove it contained no liquid explosives. Liquids and gels were banned from flights. Travelers repacked their luggage in airports, stowing all but the most necessary items in the hold.

Although plots to blow up airliners using liquid explosives are not new — such an attempt was foiled more than a decade ago — the U.S. government has been slow to upgrade its security equipment at airport checkpoints to detect explosives on passengers.

Experts said airliners are traditional targets for terrorists because they symbolize a nation's policy and tend to be easy targets that can wreak economic havoc.

"Terrorists always go after the weak link," said Charles Pena, a senior fellow at George Washington University's homeland security institute. "They saw this potential weak link in the aviation system. They're going to continue to look for potential weaknesses in aircraft security, but not to the exclusion of other targets they might be interested in attacking."