British police questioned eight people on Tuesday, at least four of them doctors, over a suspected Al-Qaeda plot to detonate car bombs in London and Scotland after the investigation spread as far as Australia.
Australian police detained an Indian doctor, Mohamed Haneef, under counter-terrorism laws. Three of the doctors being questioned trained overseas and had worked at British hospitals.
A public relations company acting for the hospital in England where Haneef worked until 2005, said a man arrested in Liverpool on Sunday was a doctor who had also worked there.
A British security source told Reuters that two of the eight suspects were Indian and the rest were Middle Eastern and "quite a few" are doctors.
The source said it was "entirely speculative" for media to suggest they formed an Al-Qaeda sleeper cell smuggled into Britain using their medical profession as cover.
"We don't know enough to say whether they were radicalized here or overseas, and how they met," he said.
The series of foiled and actual attacks poses a stern test for Prime Minister Gordon Brown, a Scot who replaced Tony Blair only last week and who has come under pressure from some quarters to change policy on Iraq and withdraw British troops.
Britain has seen a marked increase in terrorism-related attacks since the September 11 strikes on the United States and its decision to join US forces in invading Iraq in 2003.
Previous attacks, including one on London's transport system in July 2005 which killed 52 people, have mainly involved disaffected British-born Muslims, not educated professionals from overseas, security experts say.
Security sources said all those detained over this plot had links to the medical profession. Some of them are thought to be hospital workers or medical students.
A police source said they were still looking for other suspects to check they had all those who might be involved.
"It's still an active inquiry. The threat level remains at critical and until we know for sure that will probably remain the same," the source told Reuters.
One of the two men who rammed a fuel-laden vehicle into Glasgow airport on Saturday is under arrest in hospital in Scotland being treated for severe burns. Police sources said on Tuesday that he was in a critical condition.
In an investigation described as "fast-moving," police carried out a controlled explosion on a car at a mosque in Glasgow early on Tuesday. They said the car was linked to the bomb plot investigation, but stressed it had not contained explosives and had been destroyed as a precaution.
Fearing further attacks, police have banned cars and other vehicles from directly approaching airports, and security measures have been stepped up across the country. Authorities kept the threat level at "critical," the highest rating.
London police carried out a controlled explosion on suspicious objects in the Hammersmith area of west London and investigated a suspicious vehicle in the city's financial district. Both security alerts were subsequently lifted.
Australian Prime Minister John Howard said the 27-year-old Indian national seized in Brisbane had not been arrested and no charges had been made against him. Local officials said Haneef, who was registered to work as a doctor in Australia, had "some connections to the incidents in the UK."
British police said it was too early to say whether any of their officers would go to Australia to question him, or whether moves would be made to bring him to London.
The Gold Coast Hospital registrar had been recruited from Liverpool, England, in 2006 through an advertisement in the British Medical Journal.
Of the other doctors held over the plot, British police sources named one as Bilal Abdulla, who qualified in Iraq in 2004, and another as Mohammed Asha, who qualified in Jordan the same year. Asha's wife was also arrested.
In Amman, Jordan, the father of Mohammed Asha described his son as a good Muslim and dismissed suggestions that he could be involved in an Al-Qaeda-style bomb plot. "I am sure Mohammed does not have any links of this nature," he said.