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.
CBS News Correspondent Stephanie Lambidakis reports the warning is urging rail and bus industry personnel to step up security and passenger screening.
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.
Europe is also on alert. Spanish policeon a high-speed rail line between Madrid and Seville, Interior Minister Angel Acebes said.
In France, authorities last month found a bomb half-buried on a train track some 100 miles southeast of Paris andin connection with an investigation into a mysterious group's threats to bomb French railways.
The U.S. bulletin did not specify a particular city that might be targeted.
Al Qaeda 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."
Ammonium nitrate is a common fertilizer, but it can be mixed with fuel oil to make a powerful explosive.
on suspicion of being involved in a possible terrorist plot that included the discovery of 1,000 pounds of ammonium nitrate.
And police in Thailand say suspected Islamic rebels.
In addition to Oklahoma City, ammonium nitrate was used in the 2002 blast in Bali, at the World Trade Center in 1993 and in Pakistan in mid-March of this year. In years past, the Irish Republican Army used ammonium nitrate in attacks in London and in Northern Ireland.
After the Madrid train bombings, Homeland Security officials issued a bulletin advising state officials, police and transit and rail agencies to be vigilant, consider additional surveillance and to look out for unattended bags and backpacks.
Since Sept. 11, the government has issued terrorism warnings concerning a wide variety of potential targets, from fuel tanker trucks to airplanes to ships.