WASHINGTON — America is adding air marshals to overseas flights because of concerns about potential terrorism threats originating in Britain and Europe, the homeland security chief said yesterday. The Bush administration said it was satisfied with its current terrorism alert level following an attack at a Scottish airport and two foiled car bombs in London.

“I think given what we know now, we’re comfortable that we’re at the right posture,” Michael Chertoff said during a round of talk show appearances.

U.S. airports and mass transit systems are tightening security ahead of the Fourth of July holiday and more air marshals will travel on overseas flights.

“We will be doing operations at various rail locations and other mass transit locations in cooperation with local authorities. Again, not because of a specific piece of credible threat information, but because we are going into a holiday season. There will be a larger number of people traveling,” Chertoff said.

An American official, who spoke on condition of anonymity while the investigations were ongoing, said American authorities are running the names of the suspects in Britain through their databases to look for links to America.

Those checks would include watch lists such as the no-fly list; any clue that the suspects had shared an address with people in the U.S.; intelligence indicating the suspects made calls into the U.S.; and other similar types of investigative work.

It was not immediately clear if counterterrorism agencies had any hits or connections.

Airports are at the second of five security threat levels — orange — indicating a high risk of terrorist attacks. The current national threat level is yellow, or the third highest, indicating an elevated threat.

Mr. Chertoff said he does not plan to change those levels. “At this moment we don’t have a specific credible threat against the United States,” he said.

Britain has raised its security alert level to the highest possible level, indicating terror attacks may be imminent.

Mr. Chertoff said he has spoken out for some time about America’s worries involving potential terrorist threats originating in Britain and Europe.