The arrest of Al Qaeda operations chief Khalid Shaikh Mohammed could speed up terrorist attacks in the United States that are in the planning stages, the FBI said in a bulletin sent Wednesday.

The latest of the weekly FBI memos to 18,000 state and local law enforcement agencies says Mohammed's capture "deals a severe long-term blow" to Al Qaeda's ability to carry out attacks.

"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.

Federal law enforcement officials have said 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. Names of many possible Al Qaeda members, including some in the United States, also were collected.

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.