WASHINGTON - A warning that terrorists might strike trains and buses in major U.S. cities using bombs concealed in bags or luggage has the nation's transit systems ratcheting up security measures.
Greg Hull, security chief for the American Public Transportation Association, said Friday the transit systems are at "code yellow-plus" following the bulletin about a possible terror plot from the FBI and the Homeland Security Department.
U.S. officials said they had received uncorroborated intelligence reports about a plot by terrorists to target commercial transportation systems but had no information about specific cities or dates.
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.
"It should not be considered unusual that the FBI should issue this kind of a bulletin in the wake of what occurred in Madrid last month," the Amtrak passenger railroad said in a statement.
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 nationwide.
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 terror alert level remains at yellow, or elevated, the midpoint of the five-color scale. 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.
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 the Oklahoma City federal building in 1995. Both items are readily available.
The improvised bombs would 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, the government warning says.
Al-Qaida and other terrorist 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, congressional investigators say.
On Friday, Spanish authorities found and disarmed a bomb connected to a detonator with a 450-foot cable under tracks of a high-speed railway between Madrid and Seville.
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 Transportation 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 said 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 with no major new funding plans.