Daniel Gardiner, a rental agent whose company leased a Glasgow-area home searched by police, said authorities contacted his firm 10 minutes before the airport attack, saying they had tracked phone records from it linked to the foiled London car bomb attacks.

"A card was put through one of my colleague's door, asking if we would contact them," he said.

"A couple of hours later, they (police) came back to us with a name, and we were able to trace their records," he said. "The police wanted to know why we had dialed a certain phone number. They had the phone records from the situation down in London."

Ian Thomson, who lives on the quiet suburban street, said the people who lived in the house kept irregular hours: "I would see at least one of them with a stethoscope and just assumed they were doctors working shifts."

Gardiner said his tenant — the only person listed as living in the house — was thought to be a doctor at the Royal Alexandra Hospital. Bomb experts carried out two controlled explosions on a car at the hospital Sunday and Monday, Strathclyde Police said. Police said the car was linked to the investigation but that no explosives were found.

A British government security official said a loose U.K.-wide network appeared to be behind the London and Glasgow attacks, but investigators were struggling to pin down suspects' identities.

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

In a statement to the House of Commons, Smith urged Britons to remain united.

"Let us be clear: terrorists are criminals, whose victims come from all walks of life, communities and religious backgrounds," she said. "Terrorists attack the values that are shared by all law-abiding citizens.

"It is through our unity that the terrorists will eventually be defeated."