LONDON -- One day after foiling an alleged double car-bomb plot in London, Britain raised its terrorism threat assessment to its highest level yesterday after two men slammed a Jeep SUV into the departure doors of Glasgow airport, turning the vehicle into a potentially lethal fireball.
Two suspects in the airport attack were in custody, and British police later arrested two more suspects in the plots, which authorities believe were linked. The additional arrests were made in Cheshire county in northern England by officers from London and Birmingham, the Associated Press reported.
One of the Glasgow suspects was ablaze from head to foot, and as he struggled with police, "was throwing punches and shouting, 'Allah, Allah,' " an eyewitness said.
Britain's threat level was set at "critical," meaning that a further attack was considered imminent. The threat level has not been that high since last year, after authorities discovered an alleged plot to attack trans-Atlantic airliners with liquid explosives in August.
A British security official, speaking anonymously, said the heightened level reflected an assessment that the London and Glasgow attacks were "linked in some ways and, therefore, there are clearly individuals who have the capability and intent to carry out further attacks."
The links related to the way the foiled car-bomb attacks in London and the airport attack in Glasgow had been conceived and planned using vehicles and gasoline, the official said.
Within an hour of the announcement, authorities said the airport in Liverpool had also been closed until further notice, apparently reflecting a fresh area of concern as an increasingly jittery nation braced for further possible episodes.
In the United States, officials were following the events closely, but the Department of Homeland Security issued a statement from Secretary Michael Chertoff saying there were no plans to raise the national threat level.
"At this point, I have seen no specific, credible information suggesting that this latest incident is connected to a threat to the homeland," Chertoff said. Still, additional security measures were being taken at airports across nation, including increased police patrols.
Willie Rae, the chief constable of the Strathclyde area around Glasgow, called yesterday's attack an act of terrorism.
Police said one of the two suspects in the Glasgow attack had been seriously burned and had been found to be wearing a "suspicious device" when he was taken to a hospital for treatment. The hospital was evacuated, Rae said, but he declined to comment on reporters' suggestions that the assailant had been wearing a suicide bombers' belt.
The injured attacker was listed in serious condition. Police said five bystanders also were injured, but their conditions were not serious.
The attacks confronted Britain's new prime minister, Gordon Brown, with his first major crisis.
In office for only four days, a somber Brown appeared briefly on national television from 10 Downing St. late yesterday to proclaim: "I want all British people to be vigilant, and I want them to support the police and all the authorities in the difficult decisions that they have to make. I know that the British people will stand together, united, resolute, and strong."
The Glasgow attack came on the first full day of the school summer vacations, when thousands of people were awaiting flights.
The sight of the dark green Jeep Cherokee smashing into the building, its tires spinning, and bursting into flames spread panic and terror among thousands of people awaiting flights, witnesses said. Some witnesses said one of the occupants of the truck had waved a gasoline bomb.
Hours after the attack, hundreds of passengers remained stranded on airplanes on the tarmac. The authorities said they could not be moved into the airport building because of potential further dangers there.
The developments in London and Scotland deepened foreboding among security specialists that Britain was confronting a new threat: the use of relatively unsophisticated, homemade explosive devices to claim lives and spread mayhem.
Britain's newest terror alert began in the early hours Friday, when two Mercedes sedans filled with gasoline, gas canisters, and nails were found parked in the central West End theater and nightclub district.
Yesterday in Scotland, accounts by witnesses were confused, but some spoke of the two occupants of the car -- both described as men of South Asian descent -- smashing bottles of gasoline and struggling with police and others who tried to restrain them.
One of the men in the car was said to have possibly set himself on fire.
The events at Glasgow Airport came as London braced for a weekend of high-profile public events including a gay pride march, the Wimbledon tennis tournament, and a concert to honor the memory of Diana, Princess of Wales.
The police in the capital stepped up foot patrols as counterterrorism officers hunted suspects linked to the rigged cars found in London.
But the attack in Scotland seemed to have taken the authorities by surprise. Rae, the Scottish police official, said there had been no prior intelligence warning of an attack.
Brown, the prime minister who is himself a Scot, summoned two emergency meetings of the high-level security committee called Cobra to try to come to grips with the newest attacks.
In London, counterterrorism specialists suggested that the bombers who abandoned the two explosives-laden Mercedes sedans in central London may have been what a senior Western official called "less directed from Al Qaeda and more a matter of a homegrown group," although the attack seemed to be modeled on terrorist attacks in Iraq.
No one took responsibility publicly for the London plot, which was thwarted almost by accident when an ambulance crew and traffic wardens separately discovered the rigged sedans.
Several specialists and officials said the technology behind the London plot seemed to be amateurish. While the attackers apparently tried to detonate the bombs using cellphones, "they didn't go off because there were not top-grade people putting them together," the Western official said, speaking in return for anonymity because he was not authorized to brief reporters.
If the plot turns out to be the work of a small, hitherto undetected cell, that could raise alarms that Britain's terrorism threat is broader than the 2,000 suspected radicals known to authorities, said British and Western officials.
The events in Scotland seemed marked by the same sense of an improvised attack.
One witness, Lynsey McBean, said, "They were obviously trying to get it further inside the airport as the wheels were spinning and smoke was coming from them."
"One of the men, I think it was the driver, brought out a plastic petrol canister and poured it under the car. He then set light to it," McBean said, according to the Press Association.
Another witness, Scott Leeson, said the Jeep sped up to the building at around 30 miles per hour in an area where people usually drive much more slowly.
"Then the driver swerved the car around so he could ram straight into the door," the Press Association news agency quoted Leeson as saying. "He must have been trying to smash straight through. Luckily he did not get the car too far in."