Intelligence agents picked up an unconfirmed, uncorroborated report that terrorists may want to target U.S. subway systems, reports CBS News Correspondent Bob Orr. The threat was not specific to location or time, but was solid enough that the agents in the Department of Transportation felt that the subway systems should at least be warned.
The advisory asked transit system operators to "remain in a heightened state of alert."
After alerting subway systems, DOT officials expanded the warning to include commuter trains, Amtrak, freight railroads — all the rail infrastructure.
Because so many people will be traveling during the Memorial Day Weekend, the Transportation Department thought it would be prudent to err on the side of caution.
There has been nothing specific as to time, location or target. "This is another one of these very general threats that we have seen repeatedly since Sept. 11," said Orr.
Over the past week, a host of top U.S. officials have issued a series of warnings of possible fresh attacks on the United States.
Vice President Dick Cheney warned over the weekend about the probability that extremists could launch fresh attacks. FBI Director Robert Mueller said on Monday another attack was "inevitable," and told President George W. Bush this week that it would be difficult to stop another attack.
Officials said there has been a lot of intelligence coming in over the past few weeks warning of a possible attack, but they said it varied in terms of specificity and reliability.
The FBI already warned this week of possible general threats against landmarks in New York City, including the Statue of Liberty and the Brooklyn Bridge.
A source speaking on condition of anonymity said such attacks most likely would involve explosives.
Lisa Schwartz, a spokeswoman for the Metropolitan Transportation Authority, which oversees transportation in the metropolitan area, would not discuss any specific measures being taken, saying only "we're taking all the steps possible to ensure the safety of our customers and our employees."
The United States blames Saudi-born militant Osama bin Laden and his al Qaeda network for the Sept. 11 attacks, and a detained member of bin Laden's inner circle has been the source of many of the recent warnings.
Senior al Qaeda leader Abu Zubaydah, who was captured in Pakistan in March, has provided information recently that has led to alerts about possible threats to the landmarks in New York, apartment buildings, banks in northeastern U.S. states, supermarkets and shopping malls.
Officials acknowledge Zubaydah may not be telling the whole truth, but officials are erring on the sign of caution as they issue warnings.
There was no word on the source of the information for the warning on transit and rail systems.
But another U.S. official said it was not believed to be linked to Zubaydah.