A group of F.B.I. counterterrorism analysts warned this week of possible terrorist attacks in New York, Los Angeles and Chicago around Sept. 11, but officials cautioned on Thursday that they were skeptical about the seriousness of the threat.
The warning grew out of intelligence developed from an overseas source indicating that terrorists might seek to steal fuel tanker trucks in order to inflict "mass casualties" by staging an anniversary attack, officials said.
The information led F.B.I. joint terrorism task forces in Los Angeles and Newark to alert other government and law enforcement officials privately this week about the threat, law enforcement officials said. Several government officials in Washington who were briefed on the threat said it was described as credible and specific enough to warrant attention.
But other law enforcement officials in Washington and New York said that while they were aware of the warnings and were concerned about the Sept. 11 anniversary, they remained somewhat skeptical about the latest threat.
The Federal Bureau of Investigation was planning to send out another confidential law enforcement bulletin on Thursday to qualify the earlier one and emphasize that the threat of a possible tanker attack had not been verified.
"The information is uncorroborated, and the source is of questionable reliability," said Brian Roehrkasse, a spokesman for the Department of Homeland Security. "This information continues to be evaluated by the intelligence community."
There were no immediate plans to raise the national threat level, although urban transit systems remain on higher alert after last month's subway attacks in London.
Domestic security officials have long thought that tanker trucks could be used in terrorist attacks. New York, Los Angeles and Chicago are considered at the top of potential targets, along with Washington and Las Vegas, because of their size, high profiles, symbolic value and past plots by Al Qaeda.
New York City's police commissioner, Raymond W. Kelly, said in a statement that the department was aware of the threat.