GLASGOW, Scotland (CNN) --A controlled explosion has been carried out on a vehicle at the hospital treating a suspect in the attack on Scotland's busiest airport.Armed police officer observes vehicles outside Manchester Airport on Sunday.Police officers also searched several houses near Glasgow International Airport and made a fifth arrest Sunday in connection with the attack on its main terminal and a foiled car bomb plot in London.Later Sunday, Army bomb disposal experts carried out a controlled explosion on a car at the Royal Alexandra Hospital where a suspect from the airport attack was being treated for severe burns. No explosives were found.A police spokesman told The Associated Press: "It is believed that this car is connected to yesterday's incident at Glasgow International Airport."Police also raided several buildings in the town of Newcastle-Under-Lyme, a town in central England between Manchester and Birmingham. (Video Watch the latest on the investigation »)The developments came after Britain raised its terror alert status to the highest level of "critical," indicating an attack is imminent and the country's prime minister warned of dangers to come.In Scotland, officers swooped on a "number of houses" in an operation they said was linked to both Saturday's airport attack and the London bombs.In the latest in a series of arrests, one person was seized in the northwest city of Liverpool and two houses were searched. Two people, said to be a 26-year-old man and a 27-year-old woman, were earlier arrested in nearby Cheshire on the Glasgow-bound M6 highway.In the small Scottish village of Houston, just outside Glasgow, video footage showed police combing a three-bedroom rental house. Neighbors described the occupants as of "Asian" appearance.Scotland's Justice Secretary Kenny MacAskill described those behind the airport attack as "not born and bred here," indicating they were not Scottish, the UK Press Association reported.The attack in Glasgow, which according to Reuters caused five slight injuries and damaged the airport entrance, came 36 hours after two car bombs loaded with fuel, gas canisters and nails were found in central London primed to detonate.Police sources linked the airport attack to the failed London bomb plots and said they had traced the cars found in the capital back to Glasgow.Prime Minister Gordon Brown on Sunday said Britons must realize the terrorism threat their country faces is "long-term and sustained" and they must remain "constantly vigilant" about security.Brown said in a TV interview that Britons: "Will not yield, we will not be intimidated, and we will not allow anyone to undermine our British way of life."The Glasgow attack brought the airport to a standstill, with flights grounded and many passengers stuck for hours on the tarmac. The airport reopened on Sunday afternoon.Security was raised at other airports worldwide, while another alert temporarily closed Liverpool's John Lennon airport (Read how disruptions hit Glasgow after reopening)There was widespread panic at Glasgow as the flaming Jeep Cherokee smashed into the airport. Witnesses describe one of the Jeep's passengers shouting "Allah" as he fought with police and a second man set himself on fire."We tried to subdue the guy, but he's a bad boy, he's not for being subdued," said airport worker John Smitten, who ran to help police in the aftermath of the crash."He was shouting something. 'Allah,' something, 'Allah.' Every time he threw a punch, he was saying 'Allah.' "Don't miss * Witnesses tell of airport horror * Airport reopens but disruption ahead * British press fears wave of terrorismSmitten said the second man who was severely burned and "covered head-to-toe in flames" also tried to fight policemen after a taxi driver used a water hose to put out the fire.Two men were immediately arrested at the airport. One was hospitalized with severe burns; early reports that he had died were erroneous.Although witnesses earlier said a third man died inside the burning SUV, police on Sunday said a closer examination of the burned-out vehicle showed that no one was dead.In a further incident Saturday, part of Glasgow's Royal Alexandria Hospital was evacuated after a suspicious device was found on the injured suspect. The device was safely taken away and is being examined.In Washington, U.S. President George W. Bush was briefed on the situation, but officials said there was no information on increased threat to the United States and no plan to raise threat levels, although security was tightened at major airports.In announcing the UK's raised threat level, Gordon Brown said, "The first duty of the government is the security and safety of all the British people."Brown, in office just three days, said Britons stood "united, resolute and strong."In London, police were examining two cars filled with fuel, gas canisters and nails and studying CCTV footage for clues about the identities of those behind a suspected terrorist plot that could have killed hundreds. Photo See photos of the investigation in action »Officers have a "crystal clear" CCTV image of a man "staggering" from the first car after parking it outside a West End nightclub, ABC News in the United States reported. Scotland Yard refused to comment.British officials said hundreds of people could have been killed if the devices in the cars had been set off.The first car was discovered parked near Piccadilly Circus; the second was found about an hour later, less than a kilometer away near Trafalgar Square.
