LONDON — Miserable passengers pushed luggage carts through pools of backed-up storm water in tents outside Heathrow Airport on Sunday as airlines again canceled nearly a third of their flights, compounding a four-day transport backlog in London spawned by stringent new security measures.
But no letup to the exhaustive inspections was in sight, even though the government today lowered its terrorism threat level from critical to severe. British Home Secretary John Reid warned Sunday that another attempted attack in Britain was "highly likely."
"We think we have the main suspects in the particular plot. I have to be honest and say on the basis of what we know, there could be others out there," Reid told the BBC. "So the threat of a terrorist attack in the U.K. is still very substantial."
He said counter-terrorism officials were monitoring as many as two dozen "major conspiracies" and had foiled at least four "major plots" since attacks on the London transit system in July 2005 that killed 52 people.
Authorities are questioning 23 suspects in custody on suspicion of plotting to smuggle liquid explosives onto passenger jets bound for the United States. Controversial new anti-terrorism laws allow authorities to hold them for as many as 28 days without specific charges. In the meantime, authorities have declined to give further details on the detainees or their roles in the alleged conspiracy.
But British newspapers hinted at wider plots, with the Independent reporting that police think terrorist cells under surveillance possess a "huge arsenal of bombs and weapons" capable of unleashing an "apocalyptic wave of attacks."
Reid declined to discuss specifics but acknowledged that authorities thought the threat of new attacks was substantial. "This has been an ongoing threat, it is a chronic one and it is a severe one," he said.
A British anti-terrorism official said authorities had two concerns: that those under arrest could have confederates still at large who might attempt a strike, and that other, unconnected militant groups might seize the moment to attempt an attack.
"There could be as many as 20 plots being watched by the security services," the official said, who requested anonymity because investigations were ongoing. But he said he did not know of other plots considered to be an imminent threat.
The most visible sign of Britain's high alert was its airports, which for the fourth straight day were crippled by hours-long delays in passenger screening lines and backlogs in checking additional bags as a result of a wider ban on carry-on items.
Heathrow became a netherworld of chaos by Sunday afternoon, as airport officials struggling to prevent gridlock inside the terminals consigned early-arriving passengers to rain-soaked tents outside. As showers turned into heavy thunderstorms, storm drains backed up and passengers were left to wade through several inches of water, desperately seeking information about the whereabouts of their flights.
David Hashemi, a Los Angeles engineer returning from vacation in Iran, said he was told Saturday that his Virgin Airways flight was canceled.
He phoned for updates all afternoon from his hotel, only to find out later that the flight had departed on schedule, but from a different London airport, Gatwick.
"You get here and you're just walking around in the rain and don't know where to go; everyone's standing around confused," Hashemi said.
Nancy Micozzi and her daughter Kristine Tripp were camped in the second-floor departure lounge behind a mound of luggage, having defied security guards who threatened to arrest them when they barged through the front door.
"This is our second day here, and we're not scheduled to get out of here till 4 tomorrow," Micozzi said.
She said their scheduled Saturday afternoon flight to Boston was postponed twice. When passengers were finally allowed to board at 9:30 p.m., they were ordered back off the plane moments later. They were left sitting at the airport until 2 a.m., when they were bused an hour and a half to a hotel.
The airline offered no bus service to the airport the next morning and no hotel for the second night's delay. Having paid more than $100 in cab fare, Micozzi and her daughter were determined to stay at the airport until their flight left this afternoon.
"We grabbed these seats and we've been sitting here since this morning," she said. "We have no place to sleep. We are basically the homeless."
The British Airports Authority said about a third of all flights were canceled Sunday but expected "slightly less cancellations" would be required today.
But an authority spokesman, who requested anonymity, said the situation was not expected to measurably improve as long as stringent government-required inspection measures remained in place.
"It'll be going on indefinitely until the government alters their security advice," he said. "What we're dealing with is there are four times as many checks now under these new government regulations."
