The United States put its subways, buses and commuter trains on high alert after the rush-hour London bombings, moving to code orange for mass transit amid concern about a possible "copycat attack" by terrorists.

From New York to San Francisco, cities tightened security for local rail and bus lines that carry millions of Americans daily. Stepped-up safeguards included officers armed with machine guns, bomb-sniffing dogs, increased video surveillance and more police at train and bus stations.

Homeland Security Secretary Michael Chertoff (search) said Thursday that authorities had no evidence of a specific, credible threat against the United States.

However, he said, "we feel that, at least in the short term, we should raise the level here because, obviously, we're concerned about the possibility of a copycat attack."

The London attacks were well-coordinated, leading to speculation about Al Qaeda (search) involvement, and U.S. officials were trying to determine responsibility.

U.S. counterterror officials said they received intelligence last month dating back to 2004 that Al Qaeda was interested in attacking rail systems in Europe and the United States, including derailing trains or crashing trucks into them. The officials, who spoke on condition of anonymity because the intelligence is classified, said the report lacked specifics on the date and location of any potential attacks.

Security around the Capitol and at foreign embassies in Washington was increased, particularly around the British Embassy. Law enforcement authorities around the country were urged to step up security at United Kingdom diplomatic offices, and the State Department ordered U.S. embassies around the world to review their security arrangements.

"We will not yield to these people, will not yield to the terrorists," President Bush said in Gleneagles, Scotland, where he was attending the Group of Eight (search) summit. "We will find them. We will bring them to justice."

Signing a condolence book at the British Embassy in Washington, Secretary of State Condoleezza Rice (search) wrote of Thursday's bombing victims, "They will not have died in vain."

At the State Department, a British flag was run up a flagpole outside the diplomatic entrance and then lowered to half-staff by two uniformed guards.

For many Americans, the bombings and heightened security revived the tension many had felt after the terror attacks of Sept. 11, 2001, but which had seemed to recede in recent months.

"Everything is kind of vulnerable," said Bill Giel, 53, of Milford, Conn., who was riding a commuter train to work. "I just hope the authorities are doing their job in keeping things secure."

"You kind of hold your breath until rush hour is over because of the timing of the one in London," said Paul Dullea, a 37-year-old employee of the Boston Bar Association who rode the commuter rail and subway into Boston from his home in Millis, Mass.

About 29 million people in the United States take commuter trains or subways on an average workday, and millions more take buses. The New York City area accounts for about a third of the rail total, followed by Chicago, Washington, Boston and Philadelphia. The West Coast's largest transit system is in San Francisco.

The alert marked the seventh time the terror threat advisory level had been raised to orange since it was created in 2002, though, like the last time, it was for specific areas. The alert was last raised in August for financial institutions in Washington, New York and Newark, N.J., in the run-up to the November elections.

Recent intelligence has indicated that London was considered a prime target for Islamic extremists, in part because Al Qaeda was having difficulty getting people into the United States, one official said.

Two senior U.S. counterterror officials who detailed some of the classified information received in June said it indicated Al Qaeda's intention to replicate last year's Madrid train bombing attack in Europe and the United States. The officials would not specify whether the intelligence came from a person, intercepted communication or other source.

But they said the intelligence was not otherwise specific, and was dated back to shortly after the Madrid attacks in March 2004.

The officials stressed that the information came as part of a stream of reports indicating Al Qaeda's interest in transit attacks. One of the officials said a federal bulletin was not issued to private, state and local authorities because there wasn't specific information tied to an attack.

U.S. officials have long worried that terrorists would try to strike the nation's mass transit system.

Authorities are particularly concerned about transit systems in Washington, New York, Boston, Miami and Chicago because of earlier indications of Al Qaeda's interest in those cities, said John Rollins, a former senior Homeland Security intelligence official now with Congressional Research Service.

In February, FBI Director Robert Mueller told the Senate Intelligence Committee that U.S. transportation systems "remain a key target." He said last year's attacks on commuter trains in Madrid showed the devastation that a simple, low-tech operation could achieve.

U.S. security concerns and resources have focused on commercial airlines since 2001, said Sen. Susan Collins, R-Maine, who chairs the Homeland Security Committee.

"We tend to always be looking backward at what the last threat or attack was," Collins said. "As soon as we close the gaps in one area, I'm certain the terrorists will exploit in other areas."

Security efforts were not being increased at U.S. airports after Thursday's attacks.