WASHINGTON - Terrorists might try to bomb buses and rail lines in major U.S. cities this summer, according to a government bulletin issued to law enforcement officials nationwide.
The FBI and Homeland Security Department sent a bulletin Thursday night saying terrorists could attempt to conceal explosives in luggage and carry-on bags, such as duffel bags and backpacks.
The bulletin cites uncorroborated intelligence as indicating that such bombs could be made of ammonium nitrate fertilizer and diesel fuel, similar to what was used to blow up the Oklahoma City federal building in April 1995.
A senior federal law enforcement official, speaking Friday on condition of anonymity, said recent intelligence, coupled with the deadly March 11 commuter train bombings in Madrid, has increased the level of concern about a potential attack in the United States.
The bulletin did not specify a particular city that might be targeted.
Al-Qaida and other terrorist groups have "demonstrated the intent and capability" to attack public transportation with a variety of bombs, including suicide bombers, the bulletin says. Such attacks have occurred in Israel, Greece, Turkey, Spain and elsewhere.
The bulletin says that a "viable" explosive constructed of ammonium nitrate and diesel "could be concealed in standard luggage."
British authorities earlier this week arrested eight people on suspicion of being involved in a possible terrorist plot that included the discovery of 1,000 pounds of ammonium nitrate.
The warning follows by one day an FBI bulletin to state and local law enforcement agencies raising concern that terrorists might try to use cultural, artistic or athletic visas to slip into the United States undetected.
The new bulletin lists a number of suggestions for city transportation systems to enhance security. These include close monitoring of parking lots, removal of trash receptacles, limiting access points, improving lighting and beefing up overall law enforcement presence.
Barriers should be deployed at key points to prevent terrorists from parking a bomb-laden vehicle, possibly disguised as a delivery truck, close to entrances and exits.
"Question drivers and direct them to move immediately," the bulletin says.
In addition, the bulletin recommends passenger screening steps such as random security sweeps, positive matches of bags and cargo to passengers, and reminding passengers to immediately report any unattended bags or suspicious behavior.