``If individuals receive mail of which they are suspicious, they should not open it, they should not shake it,'' Ashcroft said, advising Americans to leave the area where they find such mail and call law enforcement and public health authorities.
Kenneth W. Newman, deputy chief inspector of investigations for the U.S. Postal Service, said the New York case, where a suspicious letter is being investigated as the possible source of the anthrax, would be the first time that a biological agent was sent through the U.S. mail.
The government also acknowledged Thursday it didn't know how six of the 19 suspected terrorists in the Sept. 11 hijackings made it onto U.S. soil.
``Certain information, while not specific as to target, gives the government the reason to believe that there may be additional terrorist attacks within the United States and against U.S. interests overseas over the next several days,'' the FBI said.
``The FBI has again alerted all local law enforcement to be on the highest alert, and we call on all people to immediately notify the FBI and local law enforcement of any unusual or suspicious activity,'' an FBI statement said.
President Bush said the warning was precipitated by a ``general threat'' the government received. ``I hope it's the last, but given the attitude of the evildoers it may not be,'' he said.
A U.S. intelligence official, speaking on condition of anonymity, said authorities had received an increasing amount of intelligence in the past two days about terrorists plotting to wreak more havoc through this weekend.
The possible threats ranged from diplomatic sites overseas to possible truck bombs in the United States, the official said.
Police on Thursday sharply restricted truck traffic in a 40-block zone around the U.S. Capitol.
In Houston, authorities investigated the apparent theft of 700 pounds of explosives from a storage site. Federal agents said it was too early to tell if the theft from AirJac Drilling Inc. was terrorist-related.
The warning came as Bush also disclosed that a nation formerly accused of harboring terrorists, Syria, might help with the anti-terrorism fight. ``We'll give them an opportunity to do so,'' the president said.
The government's immigration chief acknowledged that U.S. authorities don't know how six of the hijackers entered the country.
AP-NY-10-12-01 1721EDT