Police were holding eight people on Tuesday, at least four of them foreign doctors, over a suspected al Qaeda plot against Britain that triggered a manhunt reaching as far as Australia.
One British security source said two of the suspects were Indian, the rest were Middle Eastern and "quite a few" were doctors -- a contrast with recent British conspiracies led by "homegrown" militants, often with modest academic backgrounds.
Two of those arrested worked at hospitals in England, one was a doctor in Scotland and Australian police also detained an Indian doctor, Mohamed Haneef, under counter-terrorism laws. Police sources said the other suspects also had medical links.
British police are travelling to Australia to assist in the interrogation of the doctor, Australian police said on Wednesday.
The discovery of two car bombs primed to explode in London's bustling theatre and nightclub district last Friday put a city already attacked by four suicide bombers in 2005 on edge.
When a fuel-laden jeep rammed into a Scottish airport the next day, Britain's threat level was raised to its highest level, "Critical", meaning more attacks might be imminent.
Police declined comment on a Sky News report that the same people were responsible for the attempted London and Scottish bombings.
As many as four suspects in the alleged bomb plot were already known to British security services, and at least one had featured in a surveillance operation, Britain's Daily Mail newspaper reported in its Wednesday edition.
However, a counter-terrorism official in Washington said none of the suspects arrested was named on U.S. terror alert watch lists.
A scare over a suspicious bag caused chaos at London's Heathrow airport. More than 100 flights were cancelled from Terminal 4, at least seven of them to the United States, stranding passengers before Independence Day celebrations.
A security source said the bomb probe was focusing on international aspects.
He said it was too early to identify the ringleader of the alleged plot, adding: "We don't know enough to say whether they were radicalized here or overseas, or how they met."
Two more men were arrested on Tuesday in northern England but a counter-terrorism source said he understood they were not connected to the attempted attacks.
A local newspaper said they were held after 10 gas canisters, similar to those found in the cars, were delivered to a housing block.
The attacks pose a stern test for Prime Minister Gordon Brown, a Scot who replaced Tony Blair last week and has come under some pressure to withdraw British troops from Iraq.
Britain has seen a marked increase in terrorism-related plots since the Sept. 11 strikes on the United States and its decision to join U.S. forces in invading Iraq in 2003.
Muslim leaders praised the government for its "calm and reassuring tone" in handling the crisis. But fears of a backlash against Muslims in Scotland rose after attackers rammed a car into an Asian-owned shop in Glasgow and set it ablaze.
In Scotland, police have carried out four controlled explosions at a hospital linked to at least one of those arrested and at a mosque in the biggest city, Glasgow. Police said they were still looking for other suspects.
Previous attacks, including one on London's transport system in 2005 that killed 52 people, have mainly involved disaffected British-born Muslims, not educated professionals from overseas.
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, a Jordanian who qualified in the same year. Asha's wife was also arrested.