Britain intensifies hunt for al-Qaida-linked network behind recent terror attacks

British officials intensified the hunt for what they called an al-Qaida-linked network behind three attempted terrorist attacks, announcing a fifth arrest and conducting raids across a country on its highest level of alert.

Security in London was highly visible Monday morning, with long lines of cars forming behind police checkpoints on the London Bridge. Concrete car-blockers were in place protecting the Wimbledon tennis tournament.

"It is clear that we are dealing, in general terms, with people who are associated with al-Qaida," Prime Minister Gordon Brown said Sunday. He said the threat would be "long-term and sustained" but said the country would not be cowed by the plot targeting central London and Glasgow's airport.

"We will not yield, we will not be intimidated and we will not allow anyone to undermine our British way of life," he said in a televised interview.

A British government security official said a loose U.K.-wide network appeared to be behind the attacks but investigators were struggling to pin down suspects' identities _ even two arrested after they drove a Jeep Cherokee into Glasgow's main airport terminal Saturday and set it ablaze.

"These are not the type of people who always carry identity documents, or who use their real identities," the official said on condition of anonymity because of the sensitivity of the inquiries.

Home Secretary Jacqui Smith said in a series of broadcast interviews that reports about the nationalities of the suspects were "speculation."

"I think it's important that we wait until we are clear who was involved and what was and was not known," Smith said.

Residents of homes neighboring addresses being raided by police in central England and Liverpool claimed the residents were doctors or medical students.

Britain's Sky News and several British newspapers reported that two men arrested over the attacks were doctors working in British hospitals. Police in London and Glasgow refused to comment on the claim.

Two men rammed the Jeep into the airport entrance, shattering the glass doors and igniting a raging fire. One of the suspects, his body in flames after the attack, was taken to the nearby Royal Alexandra Hospital, where police on Sunday carried out a controlled explosion on a vehicle they said could be linked to the plot.

On Friday, authorities thwarted coordinated bomb attacks in central London after an ambulance crew outside a nightclub spotted smoke coming from a Mercedes that turned out to be rigged with gasoline, gas canisters and nails. A second Mercedes filled with explosives was found hours later in an impound lot, where it had been towed for parking illegally.

Late Saturday, police arrested a 26-year-old man and a 27-year-old woman on a highway in Cheshire, northern England, police said. In Liverpool, police also arrested a 26-year-old man and searched two homes.

"We are learning a great deal about the people involved in the attacks here in Glasgow and in the attempted attacks in central London. The links between them are becoming ever clearer," Peter Clarke, head of Scotland Yard's counterterrorist unit, said in Scotland.

"I'm confident, absolutely confident, that in the coming days and weeks we will be able to gain a thorough understanding of the methods used by the terrorists, the way in which they planned their attacks and the network to which they belong."

Britain raised its terror alert to "critical" - the highest possible level - and the U.S. homeland security chief, Michael Chertoff, said Sunday that air marshals would be added to overseas flights.