Airport security across the United States was beefed up Saturday in the wake of what appears to have been a third attempted terrorist attack in the United Kingdom.
In the Saturday attack that shut down Glasgow airport, a car that may have been laden with flammable liquids crashed into a terminal's doors and burst into flames, sources and officials said.
U.S. airport authorities and Department of Homeland Security officials told ABC News that heavy weapons teams, parking restrictions, increased vehicle checkpoints and other measures to step up security were being put into place at large- and medium-sized U.S. commercial airports.
Officials cautioned that there is no evidence of any specific threat inside the United States at this time, but also noted that an attack on transit is at the top of al Qaeda-linked and al Qaeda-inspired terrorist wish lists.
In the Glasgow incident, according to multiple sources, a Jeep that had aggressively evaded police checkpoints in Glasgow smashed into the front doors of a terminal. Immediately after the incident, at least two sources said it bore all the characteristics of a terrorist incident and resembled the incidents that panicked London Friday.
After the Glasgow crash, these same sources said, the driver scrambled from the vehicle with his clothing in flames. He and a passenger then struggled with police and a passerby who witnessed the incident and assisted police. They were wrestled into custody.
In the hours immediately following the incident, government officials and at least one other knowledgeable source in London were cautioning that the case did not appear to be a national security incident.
However, a police statement several hours later appeared to modify that position slightly and noted that it was simply too early to tell if the incident was linked to Friday's London discovery of two vehicles packed with incendiary materials. Finally, officials said they believed the Glasgow and London incidents were linked.
Even as U.K. officials made their initial remarks, sources say investigators were seeking to establish firm links to the two vehicles found packed with incendiary materials in downtown London early Friday that apparently were timed to explode as club goers exited for the night and headed for home.
One of those vehicles, a Mercedes 300, was spotted with a plume of liquid gas steaming from it as it sat just yards from the door of one club that holds 1,700 or more people when filled to capacity. Bomb technicians called to the scene established that it was filled with volatile materials and rigged with a sophisticated mobile phone-actuated trigger.
That car had been spotted in Scotland, sources say, in the days prior to the thwarted London attacks.
White House spokesman Tony Snow said in a phone interview shortly after the Glasgow attack that it is a terrorist attack "when someone drives a car into an airport to explode it."
But, he added, it's too preliminary to make any judgments "about the nature of whether it's al Qaeda or someone else. It could take years to determine the exact nature."
Additional security measures were taken at major U.S. airports Saturday afternoon, including the deployment of heavy weapons teams.
In New York City, extra police details had been assigned to symbolic targets and tourist locations as well as the transit system immediately following news of Friday's attempted incendiary attacks.
Snow said there are some alertness levels raised at U.S. airports, especially larger airports and possibly some medium-size terminals. He said there will be some increased inconvenience for travelers, primarily outside near curbs where TSA guards and police will be more in evidence than usual.
Snow emphasized, as he did Friday, that there is no specific and credible threat to the United States, and the national threat level remains unchanged.
He said it is important not to "get people to spin up, but it's also important to make sure everyone is vigilant."
As for President Bush, in Kennebunkport, Maine, preparing for a visit by Russian President Vladimir Putin, Snow was again at pains to emphasize that no matter where Bush is or what he is doing, he gets briefed. Today, said Snow, the president was getting briefed on Glasgow before, during and after taking a bike ride.
Snow would not discuss any increased security measures in Kennebunkport for Putin's visit, which begins Sunday.
ABC News' John Cochran, Pierre Thomas and Jason Ryan contributed to this report.