The recent arrest of a top Al Qaeda operative has confirmed for U.S. law enforcement the identifies of about a dozen suspected terrorists in this country, a government official said Thursday.

Discussing the intelligence haul from the search of Khalid Shaikh Mohammed's living quarters, this official said that authorities had known about the individuals whose names were part of the intelligence haul.

Federal law enforcement officials had said earlier that a vast amount of information was found when Mohammed was arrested at a home in Pakistan on Saturday, including computers, computer disks, portable telephones and documents.

The official, discussing the situation on condition of anonymity Thursday, said authorities were keeping the Al Qaeda suspects under surveillance and that no arrests were imminent. Continued tracking of the suspects might lead authorities to other Al Qaeda figures, the official said.

On Wednesday, the FBI warned that while the arrest was a major blow to Al Qaeda, it could speed up planned attacks by the terrorist network in the United States.

Mohammed's capture "deals a severe long-term blow" to Al Qaeda's ability to carry out attacks, said a weekly FBI memo sent to 18,000 state and local law enforcement agencies.

"However, in the short term, the apprehension may accelerate execution of any operational planning already under way, as operatives seek to carry out attacks before the information obtained through Mohammed's capture can be used to undermine operational security," the bulletin said.

The memo does not contain any mention of specific planned attacks, just a warning that it is possible some are in the planning stages.

The bulletin urges local police to remain vigilant and "alert to potential signs of terrorist planning or execution" despite the breakthrough of Mohammed's capture.

The Bush administration last week lowered the nation's risk of terrorist attack from high, or orange, to the middle level of yellow, or elevated, on a five-point scale.

Mohammed is Al Qaeda's No. 3 official and is believed to have planned the Sept. 11 attacks in New York and Washington, as well as the bombing of a synagogue in Tunisia, a planned bombing of airplanes over the Pacific Ocean and other attacks.

The FBI bulletin says that in 2002 Mohammed met with Jose Padilla, a U.S. citizen now in federal custody on charges that he planned to detonate a radiological "dirty bomb" in the United States.

Arrested with Mohammed on Saturday was Mustafa Ahmed al-Hawsawi, who the FBI says was a key financier of the Sept. 11 attacks.