Mayor Thomas Menino said he would not specify what he and Ashcroft discussed in the Thursday morning call. Public safety officials were unable to corroborate the threat, he said.

''We couldn't find any basis for it,'' the mayor said Friday.

Boston FBI spokeswoman Gail Marcinkiewicz said the FBI has investigated and discredited the threats, but she could not comment on why Ashcroft warned local officials.

''The information upon which these reports are based have been analyzed and evaluated, and the threats do not appear to be credible,'' Marcinkiewicz said.

Acting Gov. Jane Swift refused to comment on her discussion with Ashcroft or any specific security precautions.

Authorities have said that Sept. 22 - Saturday - emerged as an important date in evidence found during investigations into the hijackers. They say intelligence intercepts, witness interviews and evidence gathered in hijackers' cars and homes indicated a second wave of violence was planned.

Authorities have evidence that at least four people targeted by the terror investigation were booked on flights Saturday leaving San Antonio for California or Denver.

Security in Boston and other cities has been increased since the Sept. 11 terror attacks in New York and Washington. A few days after four jetliners were hijacked and crashed, the FBI warned Atlanta, Richmond, Va., and Boston about possible strikes. But those warnings were rescinded.

On Thursday, the FBI issued new warnings to local law enforcement to be on guard for attacks. Authorities said the warning was not based on any evidence of a direct threat but rather on raw information.