WASHINGTON -- Trains and buses in major U.S cities may be targeted this summer by terrorists using bombs hidden in bags or luggage, federal counterterrorism officials have told law enforcement and transportation officials in a nationwide bulletin.

FBI and Homeland Security Department officials said they had received uncorroborated intelligence reports about a plot by terrorists to target commercial transportation systems. The bulletin, issued late Thursday, mentioned no specific cities or dates and did not elaborate on the source of the information.

A senior federal law enforcement official, speaking Friday on condition of anonymity, said the intelligence, coupled with the deadly March 11 commuter train attacks in Madrid in which bombs went off inside backpacks, has increased the level of wariness about a similar attack in the United States.

Amtrak spokesman Dan Stessel said the company stepped up security after the Madrid bombings, including use of bomb-sniffing dogs, although the company's trains have received no specific or credible threats.

Homeland Security spokesman Brian Roehrkasse said information in the bulletin was being shared via the National Law Enforcement Telecommunications System to ensure proper security measures are implemented around the country.

Officials said the message was sent mainly out of an abundance of caution, and the threat -- deemed "somewhat credible" by one official -- was not causing undue alarm throughout the government.

The nation's color-coded terror alert level remains at yellow, or elevated. It was last raised to orange, or high, on Dec. 21 amid suspicions about terror attacks using commercial aircraft. The level returned to yellow on Jan. 10.

Passengers could see changes because of the bulletin. Federal officials are encouraging local transit authorities to conduct random passenger inspections and security sweeps of stations and to increase public announcements encouraging people to report unattended baggage or suspicious behavior.

Lynn Brown, spokeswoman for Greyhound Lines Inc., said at its Dallas headquarters that the bus company tells employees to be extra vigilant when such alerts are circulated. She said, however, that the sort of passenger inspection methods used by airports are impractical at the company's 3,600 locations, with 18,000 departures daily.

Intelligence indicates a plot might involve bombs made of ammonium nitrate fertilizer and diesel fuel, similar to the explosive concealed in a rental truck that blew up an Oklahoma City federal building in 1995. Both items are easily available around the country.

These improvised bombs would then be concealed in luggage and carry-on bags, such as backpacks or duffel bags, and detonated either aboard buses or trains or in transportation stations, according to the government warning. A viable explosive could be concealed in luggage, it says.

Al-Qaida and other terror groups have "demonstrated the intent and capability" of attacking public transportation systems using a variety of bombs, the bulletin says. Attacks in Israel, Greece, Turkey, Spain and elsewhere have used suicide bombers or triggered bombs with timers and cell phones.

Between 1997 and 2000, more than 195 terror attacks occurred on transit systems worldwide, according to congressional investigators.

In Spain on Friday, police found a bomb connected to a detonator with a 450-foot cable under tracks of a high-speed railway between Madrid and Seville. Disposal experts disarmed the bomb.

British authorities arrested nine people this week on suspicion of having links to a possible terror plot that involved 1,000 pounds of ammonium nitrate. It is unclear whether that alleged plot involved a public transit attack.

More than 9 billion trips are taken each year on the U.S. public transportation system, with 32 million trips every weekday -- about 16 times the number of trips taken on airlines, according to the American Public Transit Association.

The association estimates that $6 billion is needed to upgrade and modernize U.S. transit systems to meet security needs. The Transportation Security Administration dedicated only $10 million for passenger rail and public transit security in the current year's budget, according to the House Homeland Security Committee.

"Failure to invest in the security of passenger rail and public transit could leave these critical systems vulnerable to terrorist attack," the committee's Democrats wrote in a letter to Homeland Security Secretary Tom Ridge. "Making these systems as safe as they can be from terrorist attack must be a high priority."

After the Madrid bombings, the Homeland Security Department announced a series of security initiatives, but no major new funding plans were proposed.