LONDON – U.S. airport security will be even stiffer today as British security forces continue to hunt for suspects in an alleged terrorist plot to blow up as many as 10 transatlantic jetliners in flight between Great Britain and the United States.

Airline passengers at all major U.S. airports will pass through double screening starting today, first at the security checkpoint and again at the boarding gate, to make sure they’re not carrying liquids onto planes. U.S. authorities banned airline passengers from bringing liquids or gels onto airliners Thursday after British authorities arrested 24 men suspected of involvement in the terrorist plot.

The suspects were British citizens, mostly of Pakistani descent. Though British officials expressed confidence that the key players in the plot were in custody, they said that more suspects could be at large.

British and U.S. officials said the suspects were aiming to smuggle liquid explosives, timers and detonators onto flights in carry-on luggage. They could assemble bombs from those parts in flight.

Homeland Security Secretary Michael Chertoff said the plotters had “accumulated and assembled the capabilities” to carry out a mission that was in “the final stages of planning before execution.”

Chertoff described the plot as “about as sophisticated as anything we’ve seen in recent years as far as terrorism is concerned. This was not a situation with a handful of people sitting around dreaming about terrorist plots.”

Chertoff said the plot had all the earmarks of al-Qaida. “It was sophisticated, it had a lot of members and it was international in scope. It was, in some respects, suggestive of an al-Qaida plot.”

However, both Chertoff and FBI Director Robert Mueller declined to say with certainty whether the terrorist organization was involved.

Chertoff also warned that there was “sufficient uncertainty about whether … the British have scooped everybody up.”

“It doesn’t mean we know for a fact there are people out there who are still active, but as anybody who has been involved in these investigations knows, we’re going to learn more things and the British are going to learn more things in the next hours and days,” he said. “And given the amount of planning and effort that was put into this plot, I think it would be a little bit risky to assume that everything is shut down and the threat has gone away.”

British Home Secretary John Reid said that the terror threat to the public was the biggest that Britain had ever faced. Police said that the alleged plot had a global dimension and that security services were cooperating with foreign counterparts.

U.S. and United Kingdom officials declined to reveal publicly the dates, times or destinations of the flights allegedly targeted, though officials privately identified the targets as United, Continental and American Airlines nonstop flights from England. British Airways also may have been targeted.

A senior U.S. official, however, speaking only on the condition of anonymity because the information is part of a law enforcement investigation, said the plotters were planning to blow up planes headed from London to Boston, Chicago, Los Angeles, Miami, New York, Washington and possibly other cities. The official said the flights appeared to have been chosen because the destinations are popular, the flights were expected to be full and because they were leaving within 60 to 90 minutes of one another, which would have allowed the terrorists to detonate their bombs over the Atlantic simultaneously.

Another U.S. official familiar with the plot, speaking on condition of anonymity because British authorities were handling the information, said it was conceivable that the suspects could have “launched something within a month.”

That would be within range of the fifth anniversary of the Sept. 11, 2001, attacks, which killed nearly 3,000 people when terrorists hijacked domestic jetliners and crashed them into the World Trade Center, the Pentagon and a field in Pennsylvania.

If the foiled plot had been successful, it could have rivaled Sept. 11 in scale. Typically, airlines use jumbo jets on international flights, such as the Boeing 747, which seats about 416 passengers, and the Airbus 310, which holds more than 370 passengers.

“If they managed to pull this off, it would have been spectacular,” said Peter Lang, former head of the Middle East intelligence section at the Defense Intelligence Agency. “They would have killed several thousand people.”

President Bush, traveling from his Texas ranch to Green Bay, Wis., said the foiled plot serves as a stark reminder that the United States “is at war with Islamic fascists who will use any means to destroy those of us who love freedom.”

He said that America is safer since the Sept. 11 attacks, but added, “Obviously, we’re still not completely safe, because there are people that still plot and people who want to harm us for what we believe in.”

Highest level

Even as they broke up the plot, British officials increased their terror alert to its highest level, meaning that an attack is imminent.

U.S. officials followed suit, increasing the threat level to red, America’s highest alert, for flights from Great Britain, and to orange, the second-highest level, for all other flights. A “code red” alert requires airlines to give the government a list of passengers aboard affected flights.

The safety precautions threw U.S. and British airports into turmoil. Long lines formed behind inspection stations and many flights were delayed nationwide.

“It’s a mess … no one knows what’s going on, and I don’t mean the passengers, I mean the security people,” said Joan Caton Cromwell, a Washington, D.C., resident who was en route to San Jose, Calif., via San Diego.

At Charlotte-Douglas International Airport in North Carolina, eight people stood in a corner of the terminal with their heads bowed in prayer. Ralph Sauers, his wife, Maridee, and their children, Luke and Sara, of Fort Mill, N.C., were headed on a three- to four-year mission trip to Africa.

“We’d have prayed anyhow,” said Ralph Sauers Sr. “We were a little more emphatic today.”

Thursday evening, the Air Transport Association, the trade group for major airlines, said that tougher security measures would be in place nationwide starting Friday that could disrupt normal travel.

“It is reasonable to expect longer security lines and delayed flights,” the group said in a statement. “Passengers need to check with their airline on a regular basis and arrive at the airport significantly earlier for their flight. It would be a good idea to keep carry-on luggage to a minimum.”

The association described the measures as temporary and said they’d remain until the government deemed them no longer necessary.

Douglas Laird, an aviation expert and former security chief for Northwest Airlines said the plot described Thursday resembled a 1994-1995 attempt, code-named “Bojinka,” to blow up a dozen airliners simultaneously over the Pacific Ocean using liquid explosives smuggled onto planes in contact lens-solution bottles. That plot was allegedly masterminded by 1993 World Trade Center bomber Ramzi Yousef.

Several national security experts said the foiled attempt underscores the fact that al-Qaida and affiliated terrorist groups are still potentially capable of unleashing destruction on a grand scale.

Ben Venzke, CEO of IntelCenter, a Washington-area private contractor that provides counterterrorism support to the intelligence community, said that the thwarted attack is consistent with al-Qaida’s intentions to make a future attack against the West bigger than the last.

The terrorist group’s rationale, he said, would be to show the world that al-Qaida doesn’t have “a diminished capacity” despite five years of the U.S.-led war on terror.

“Whatever they would be planning next in terms of an attack on the United States would exceed or be equal to what had been done before … to show that they are still capable,” he said.

Former defense official Lang said that the al-Qaida network characteristically plots “two or three operations at the same time” and may be poised to make another strike.

But Bill Rosenau, a terrorism and counterinsurgency expert with the Washington office of the RAND Corp., an international think tank, said it’s too early to accurately assess the scope of the plot.

“There’s so much about this case that we don’t know yet,” he said. “We don’t know how capable these guys really were or whether this was just very wise pre-emption on the part of the British.”

While Bush hailed British officials for preventing the attack, several critics of the Bush administration’s handling of national security said the plot exposed how little progress the United States has made in domestic security since Sept. 11.

“Catching potential terrorist attacks at the last minute should not be a point of pride, but rather a point of departure,” said Rand Beers, a former National Security Council aide and special assistant to Bush on counterterrorism. “More must be done to improve our homeland security. When it comes to being prepared, the United States is still at code red.”