Talc is the substance found in a threatening letter sent Thursday to the Senate majority leader's Capitol office, the FBI said Friday

The letter -- a copycat mailing of earlier sent anthrax letters -- arrived just before noon Thursday to Sen. Tom Daschle's office.

"We are going to investigate the letter as a criminal hoax," FBI spokesman Chris Murray said.

At the time of the letter's discovery, field agents conducted two tests on the letter, said U.S. Capitol Police spokesman Lt. Dan Nichols. Both came back negative.

The letter was handed over to the FBI who sent it to Ft. Detrick military base in Maryland for further testing.

U.S. Capitol Police have been on alert for any suspicious materials following the Sept. 11 terror attacks and subsequent mailings of at least two anthrax-laden letters to the Senate, one of which arrived at the personal office of Daschle, D-S.D., on Oct. 15, forcing the shutdown of the Capitol complex two days later.

This time, the letter was opened in the Capitol itself, inside Daschle's leadership office.  Daschle was not present at the time, his spokeswoman said.  The U.S. Capitol physisician was called to the office just in case, but all the staff was safe following the letter's opening.

The postmark established that the letter was sent from London though the date it was mailed was blurred.  The letter said it contained anthrax, "death to America, something to that effect, and 'stop the bombing' was the only phrase that was new," said Daschle Thursday.  The last statement was considered a reference to the U.S.-led war in Afghanistan.

The letter's arrival shut down the building briefly Thursday. Visitors to the Capitol were denied entry and the floor above the senator's office was closed, Nichols said.  The building reopened shortly afterward. The building was never evacuated, he said, but no new visitors were allowed in.

"We didn't let anybody in, didn't let anybody out," Nichols said.

A hazardous materials emergency response team arrived on the scene, but Nichols said there was little fear of new anthrax mailings entering the Capitol since all mail is being irradiated before it is sent to the Capitol complex.

The Associated Press contributed to this report.