British police are investigating the possibility that the same two men who left car bombs in London also drove a flaming SUV into a Glasgow airport terminal the next day.

So far, however, the police have few hard answers for the public about the incidents that have triggered a security crisis in the U.K.

One clear thread that has emerged is that most of the suspects are either physicians or have some type of medical background.

British police focused Tuesday on at least five doctors and a medical student in their investigation.

Among those arrested was an Indian Muslim doctor in Australia -- Mohammed Haneef.

Australian officials arrested Haneef Monday as he tried to board a flight with a one-way ticket out of the country. Officials say Haneef was a foreign doctor, born in India but recruited to Australia from Liverpool, England.

Haneef worked at the same hospital in England as another man arrested in the case -- a 26-year-old who hasn't been named yet.

Halton Hospital spokesman Mark Shone said Tuesday that Haneef worked at the facility in 2005 as a temporary, on-call doctor. 

Shone confirmed that the 26-year-old man arrested Saturday in Liverpool had also worked at the hospital. He would not provide the man's name.

Bilal Abdulla, the Iraqi-born doctor arrested at the scene of the Glasgow incident, is reportedly friends with Mohammed Jamil Abdelqader Asha, a Jordan-born physician who had been working near Newcastle-on-Lyme.

Asha and his wife, a hospital laboratory researcher, were arrested Saturday on a highway in northern England.

Abdulla worked at the Royal Alexandra Hospital near Glasgow. Khalid Ahmed, a Lebanese doctor, is alleged to be the SUV's driver. Two men aged 25 and 28 arrested Sunday at residences at the hospital were identified as a junior doctor and medical student.

Some who know him say Abdulla had voiced support for Iraqi insurgents in the past.

"He was very angry about the West, particularly Britain and America, and obviously the invasion of Iraq," said Shiraz Maher, a friend.

Some who know Asha say he was a good Muslim with liberal thinking.

Investigators haven't yet determined a link between two Indian and the six Middle Eastern-born suspects arrested or detained so far.

Security scares

Since the discoveries, Britain's security threat level has been set at "critical."

Police arrested two men Tuesday attempting to buy gas canisters in Blackburn, northern England. However, they can't say if the men are connected to the existing suspects.

A bomb disposal team conducted a controlled explosion on a vehicle outside a Glasgow mosque Tuesday.

Police said that there was no indication of a link between Tuesday's incident and the weekend attacks.

Terminal 4 at London's Heathrow airport reopened Tuesday after a security scare led to the evacuation of thousands of passengers following the discovery of a suspicious package.

Officials confirmed that a bag was found and that passengers were being searched.

Police also ordered the evacuation of the departure lounge so that all passengers could be searched a second time.

The terminal reopened shortly after 5 p.m. local time.

British Airways said it had cancelled all its European departures from the terminal for the day, as well as all long-haul flights due to depart between 3 p.m. and 9 p.m. local time.

Canadians told to be 'cautious'

Meanwhile, Canada's Department of Foreign Affairs is urging Canadian travellers in the United Kingdom to be cautious.

Foreign Affairs spokesperson Catherine Gagnaire says there is no official travel warning for Canadians at this time.

However: "Canadians should exercise caution and maintain a high level of personal security awareness at all times and in all places" and keep on top of developments, reads the department's latest travel report for the U.K.

With a report from CTV's Jed Kahane and files from the Associated Press