Doctors from Iraq and Jordan were identified Monday as two of the seven suspects in custody in Britain's failed car bomb attacks.
Police bolstered security at British airports, train stations and on city streets, using armed officers and stop-and-search powers to check over suspect cars, and said their intensive inquiry was also spanning overseas.
A British government security official, speaking on condition of anonymity because of the sensitivity of the case, said several other countries - including Pakistan - had been asked to check possible links with the suspects.
British-born terrorists behind the deadly 2005 London transit network bombings and a host of thwarted plots have been tracked attending terror training camps and meeting foreign radicals in Pakistan.
"We have asked partners overseas to check possible links and that work has begun," the security official said.
In Britain, Home Secretary Jacqui Smith said police had searched at least 19 locations as part of a "fast-moving investigation."
It is a time of already heightened vigilance, less than a week before the second anniversary of the deadly London transit bombings on July 7, 2005.
In the latest attacks, two car bombs failed to explode in central London on Friday and two men rammed a Jeep Cherokee loaded with gas cylinders into the entrance of Glasgow International Airport and then set it ablaze Saturday.
Police in Glasgow said that two men were arrested Sunday in the airport bomb attack investigation, meaning seven suspects are now in custody.
Strathclyde police said the two men, aged 25 and 28, had been detained under anti-terror laws.
The unidentified driver of the Jeep, which burst into flames, is being treated for serious burns at Royal Alexandra Hospital in Glasgow, where he is under arrest.
Bilal Abdulla, an Iraqi doctor who worked at the same hospital, was the other man arrested at the airport and being held at a high-security police station in Glasgow, authorities said.
According to the British General Medical Council's register, a man named Bilal Talal Abdul Samad Abdulla was registered in 2004 and trained in Baghdad.
Staff at the Glasgow hospital said Abdulla was a diabetes specialist, but offered no other details.
The government security official said investigators were working on one theory that the same suspects may have driven the explosives-laden cars into London and the blazing sport utility vehicle in Glasgow.
A second man arrested late Saturday on a highway in central England is another doctor - Mohammed Jamil Abdelqader Asha, police officials said. A Jordanian official said Asha was a native Palestinian who carried a Jordanian passport.
Britain's Sky News reported a third suspect being held was also a doctor, but gave no details and police would not confirm the report.
"This case could be the final proof that an idea those involved in these type of attacks are all young, angry and poorly educated is a mistake," said Dr. Paul Cornish, an ex-army officer and director of defense studies at London's Chatham House.
"It's wrong to suggest al-Qaida are ignorant hill men, they are often middle or upper-class and well educated."
Salil Vengalil, a doctor at North Staffordshire Hospital, near the Midlands town of Newcastle-under-Lyme, said Asha worked in the neurology department but he did not know when he started on the job there.
A doctor at the Royal Alexandra Hospital in Glasgow, who refused to give his name, said he recognized Asha as a doctor who kept an office in his hospital as well, but the hospital would not confirm that he did.
In Jordan, Asha's brother Ahmed told The Associated Press his 26-year-old sibling "is not a Muslim extremist, and he's not a fanatic."
Former U.S intelligence officer Bob Ayers, now a security analyst based in London, said wealth or intelligence matters little to people committed to extremism.
"We shouldn't be surprised that educated men are as involved as poor youngsters," Ayers said. "They all subscribe to the same radical ideology, that's the only criteria they need to fill."
Information also surfaced Monday suggesting authorities had been close on the trail of the airport bombers before the attack.
Real estate agent Daniel Gardiner, whose company leased a Glasgow-area home searched by police, said officers contacted his firm just before the airport blaze, saying they had tracked phone records linked to the property.
Officials recovered at least one cell phone from the car bombs in London, authorities said.
Police in London flooded subway and train stations as part of a huge show of force - even clamping down on access to the capital's annual Wimbledon tennis tournament, where concrete vehicle blocks were placed in front of the main entrance.
At the Glasgow hospital, bomb experts carried out two controlled explosions on a car at the hospital Sunday and Monday, Strathclyde Police said. Officers said the car was linked to the investigation, but that no explosives had been found.
Representatives of the Israeli Defence Ministry responded to recent reports about the possible delivery of S-300 SAM systems from Russia to Syria. Israeli Defence Minister Avigdor Lieberman said that Israel would destroy those systems
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