WASHINGTON (AP) _ With America already on heightened alert, Attorney General John Ashcroft said Friday citizens should be cautious in opening suspicious items they receive in the mail.
The new warning was prompted by another anthrax scare in New York and came one day after the FBI issued its most stark and specific warning Thursday about the possibility of more terrorist attacks.
``If individuals recieve mail of which they are suspicious, they should not open it, they should not shake it,'' Ashcroft said, advising Ameicans 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.
Meanwhile, the government's immigration chief acknowledged that U.S. authorities don't know how six of the hijackers entered the country.
``Six of the individuals, we can find no record of them period. That's not just INS, that's everywhere,'' said James Ziglar, head of the Immigration and Naturalization Service.
Ziglar said 13 hijackers had entered the United States legally, but three had overstayed their visas: Nawaf Alhazmi, Waleed M. Alshehri and Ahmed Alghamdi.
A fourth, Hani Hanjour, had been in the United States legally at various times for the past decade, but immigration officials said they were unable to determine whether he was here lawfully on the day of the attacks.
Ziglar underscored the government's doubts over the identities of some of the hijackers.
``I suspect one of the reasons the FBI issued the pictures and the names a week or so ago was to find out if anybody out there knew whether this person was the person who has the name,'' the commissioner told a House panel.
``It's a problem not knowing who these people were and being able to match these names with faces,'' Ziglar added.
The FBI's warning was the second this week. On Sunday it asked law enforcement to move to its highest state of alert.
Bush, Attorney General John Ashcroft and FBI Director Robert Mueller have said they intend to alert Americans to any credible threats.
In recent days, the FBI has asked supervisors of water systems, nuclear and electric power plant operators, owners of crop dusters and drivers of hazardous waste trucks among others to increase security to ward off attacks.
In other developments:
_A law enforcement official said traces of anthrax had been found in the mailroom at the American Media building in Florida where one employee contracted the disease and died. One of two other employees who inhaled anthrax in their nasal passages, Ernesto Blanco, 73, works in the mailroom.
_After meeting in Washington with U.S. Customs Commissioner Robert Bonner, Canada's Customs and Revenue commissioner said his country does not think ``we are a source'' of transit for any of the 19 hijackers. ``There is no evidence that they did'' visit or enter the United States through Canada, Rob Wright said.