Britain raised its terror alert to "critical" — the highest possible level — and the Bush administration announced plans to increase security at airports and on mass transit.

In Honolulu, Department of Transportation spokesman Scott Ishikawa said that security would be "beefed up" at the Honolulu International Airport's perimeter and curbside areas, but there would be no additional security checkpoints implemented.

In the Scotland attack, one of the men in the car was in critical condition at a hospital with severe burns, while the other was in police custody, said Scottish Police Chief Constable Willie Rae. He said a "suspect device" was found on the man at the hospital and it was taken to a safe location where it was being investigated.

Rae would not say whether the device was a suicide belt. The green Jeep barreled toward Glasgow's main airport terminal shortly after 3 p.m. Officials said bollards — security posts outside the entrance — stopped the driver from driving into the bustling terminal, but the nose of the vehicle smashed the glass doors.

British security officials said evidence pointed toward the Glasgow attack being a suicide mission.

"I can confirm that we believe the incident at Glasgow airport is linked to the events in London yesterday," Rae said. "There are clearly similarities and we can confirm that this is being treated as a terrorist incident."

Police foiled the plot Friday after two cars were found in central London packed with explosives — one outside a nightclub near Piccadilly Circus and another parked nearby.

A British government security official said the methods used in the airport attack and Friday's thwarted plots were similar, with all three vehicles carrying large quantities of flammable liquid.

Lynsey McBean, a witness at the terminal, said the driver kept trying to push the car forward after it got stuck, and "the wheels were spinning and smoke was coming from them."

She said one of the men then took out a plastic gasoline canister and poured a liquid under the car. "He then set light to it," said McBean, 26, from Erskine, Scotland.

Police subdued the driver and a passenger, both described by witnesses as South Asian — a term used to refer to people from India, Pakistan, Afghanistan and other countries in the region. A previous round of terrorist activity in Britain, in July 2005, was largely carried out by local Muslims, raising ethnic tensions in Britain.