British police arrested a fifth person on Sunday after a fuel-loaded jeep was rammed into Scotland's busiest airport in what police said was a terrorist attack linked to two failed car bombings in London.
The mode, timing and targets of the three attacks suggested a campaign linked to Prime Minister Gordon Brown's taking office earlier this week, security sources said. Brown himself said he saw a direct link to radical Islamists.
"It is clear that we are dealing in general terms with people who are associated with al Qaeda," Brown, a Scot who took office only last Wednesday, said in a television broadcast.
A police source said there was reason to believe those arrested were not British citizens. They included a 26-year-old man and a 27-year-old woman seized on a major highway in northern England on Saturday night and another man, 26, detained in Liverpool, 355 km south of Glasgow, on Sunday.
Two more men, described by witnesses as Asians, were taken into custody on Saturday immediately after they slammed a Jeep Cherokee into Glasgow airport and set the vehicle ablaze.
Most of the Asian population in Britain comes from the sub-continent, including India, Pakistan and Bangladesh.
Authorities evacuated a terminal at Heathrow airport to investigate a suspect package but later reopened it after declaring the area safe. Police also carried out a controlled explosion on a suspicious car parked at a hospital near Glasgow.
They said the car, near the hospital where one of the two Glasgow airport assailants was being treated for severe burns, was linked to the attack but not thought to contain explosives.
The airport attack came barely 36 hours after two car bombs loaded with fuel, gas canisters and nails were found in central London's theatre district primed to detonate.
The police source said multiple leads were still being followed but would not specify how many suspects still at large.
Following the series of threats, Britain raised its national security level to "critical", meaning the risk of further attacks was imminent, and increased security at airports.
"We are dealing with a long-term threat. It is not going to go away in the next few weeks or months," Brown said.
London's counter-terrorism chief inspected the Glasgow crime scene on Sunday and said the links between the attacks were "becoming ever clearer". He did not elaborate.
The United States also ramped up transportation security, including more air marshals on flights to Britain amid concern of copy cat strikes.
Outside Glasgow, Scotland's biggest city, police in white body suits searched houses near the airport. Neighbours said two Asian men had moved into a house there a month ago but had kept very much to themselves.
"I don't remember seeing them at all," said Mae Gordon, 67. "They were the only people around here you would never see."
Britain has seen an increase in terrorism-related threats since the Sept. 11 strikes on the United States and since it joined U.S. forces in invading Iraq in 2003. Some analysts believe the latest attacks may be designed to exert pressure on Britain to withdraw its troops from Iraq and Afghanistan.
"ACT OF EVIL"
Brown, facing pressure from some quarters at home to change policy on Iraq and withdraw British troops, appeared on BBC television on Sunday to discuss events.
"Irrespective of Iraq, irrespective of Afghanistan, irrespective of what is happening in different parts of the world, we have an international organisation trying to inflict the maximum damage on civilian life in pursuit of a terrorist cause that is totally unacceptable to most people," he said.
"Terrorism can never be justified as an act of faith. It is an act of evil in all circumstances."
In the attack in Glasgow, 600 km north of London, witnesses said two men raced a green Jeep Cherokee into the glass doors of the airport terminal before dousing it in petrol and setting it alight in a giant fireball.
Security analysts said the earlier London car bomb plot bore the hallmarks of a previous al Qaeda plan to attack London with fuel-filled cars, and another plan to bomb a major night club.
The series of plots comes almost two years since the July 7, 2005 suicide attacks on London's transport system, when four British Islamists killed 52 commuters.
(Additional reporting by Luke Baker, Mark Trevelyan and David Clarke)