Homeland Security Secretary Michael Chertoff lowered the threat level for the nation's mass transit and ferry systems last night, concluding a 36-day period of high alert after the train and bus bombings in London prompted fears of a copycat attack in the United States.

Effective at 8 p.m. local time or after evening rush hours, state and local operators of bus, train, subway and passenger boat systems were permitted to relax security measures rushed into place after last month's deadly attacks. Transit officials in Washington and New York said they will maintain some precautions, such as random bomb sweeps and police patrols.

In a written statement, Chertoff said that "there is no specific, credible intelligence information indicating that an attack in the United States is imminent."

But because the July 7 attack in London and one in Madrid in March 2004 "were conducted without warning," transit systems, especially larger ones, "will maintain a strengthened baseline level of preparedness beyond what existed before," he said.

U.S. authorities raised the color-coded threat level from yellow, or elevated, to orange, or high, after four suicide bombers killed 52 people in London. The alert was extended after a failed bomb plot July 21 before returning to yellow yesterday.

The attacks, along with last year's commuter rail blasts in Madrid, which killed 191 people, underscored the inherent vulnerability of public transit. U.S. authorities described the recent alert and ongoing efforts as an attempt to inject at least some unpredictability into the security profiles of open systems that transport 32 million people a day.

A recent survey by the University of California at Los Angeles's Institute of Transportation Studies found that 85 percent of 113 U.S. transit systems have conducted threat studies and widely believe that more police, surveillance technology, public education and better design would be "somewhat effective." However, institute director Brian D. Taylor said, "even combining these solutions would leave systems with vulnerabilities they just don't see how they could close."

Washington's Metro system announced that transit police and bomb-sniffing dogs would continue random sweeps of trains, buses, rail yards and garages throughout the capital area.

Authorities also urged customers "to remain vigilant to keep the system safe."

"We're still going to do some random sweeps," Metro spokesman Steven Taubenkibel said.

The District, U.S. Capitol and U.S. Park Police departments also will continue increased patrols of critical facilities, stations, buses and bridges.

Restrictions on taking leave, however, will be eased for about 400 Metro Transit Police officers. And the amount of overtime allowed for the officers, which has cost about $10,000 a day, will be reduced. Nationwide, transit operators estimated the Code Orange alert cost at least $900,000 a day.

New York authorities plan to continue random searches of commuters on the city's subway system, according to police.

National counterterrorism officials yesterday sought to play down an alert sent out this week by several FBI offices. They had warned of possible attacks involving fuel tanker trucks in Los Angeles, New York and Chicago around the Sept. 11 anniversary.

Numerous officials said that the information came from a lone, unreliable source and that the tip could not be corroborated. An FBI official said yesterday that the source was overseas and had recanted his allegations.

The FBI was preparing a special bulletin for law enforcement agencies to rebut the information, officials said.

But authorities also defended its limited release.

"We feel we have the obligation to share the information," said Robert B. Stephan, acting undersecretary of Homeland Security, echoing an FBI official who declined to be named because the investigation is ongoing. "We pass that information out to everyone [with the caveat] that this source is very, very questionable in terms of credibility. It's not being corroborated."

Staff writers Sari Horwitz and Debbi Wilgoren contributed to this report.