The latest case comes after a letter containing a white powdery substance that tested positive for the potentially deadly bacteria anthrax was sent to Senate Majority Leader Tom Daschle.
The letter was received at Daschle's personal office in the Hart building at approximately 10:30 a.m. today and was opened and handled by one of the South Dakota Democrat's interns and a member of his staff.
Two preliminary field tests performed by Capitol Police indicated the presence of anthrax spores.
President Bush disclosed the incident this afternoon after speaking on the phone with the Senate leader.
"His office received a letter, and it had anthrax in it," the president told reporters during a photo op with Italian Prime Minister Silvio Berlusconi in the White House Rose Garden. "He said it was a letter that had been wrapped a lot and that the powder was within the confines of … the envelope."
Sources said the letter was wrapped in tape and contained a note that read: "This is anthrax."
Daschle's office has been quarantined and between 40 and 50 people — including members of his staff and other personnel who may have come in contact with the letter — are being tested and treated with antibiotics for possible anthrax exposure.
"They are innocent people caught up in a matter [with] which they have nothing to do," Daschle, who was not in his office when the letter was received, said at a news conference. "I am very, very disappointed and angered."
All mail deliveries to Capitol Hill have been suspended and unopened letters and packages received by House and Senate offices were being collected as officials put new screening procedures in place. Tours of the Capitol building have also been canceled indefinitely.
Daschle, visibly upset by the incident, vowed Congress' work would not be disrupted.
"This Senate and this institution will not stop," he told his colleagues on the Senate floor. "We will not cease our business … Our work will continue."
At least a dozen reports of suspicious packages were phoned in by the offices of various lawmakers on Capitol Hill today. Capitol Police spokesman Lt. Dan Nichols said the letter received by Daschle's office was the only one that tested positive for any harmful substance. He stressed that the tests were only preliminary.
"We've run a number of calls today and this is the only one we've had a preliminary positive indication on a field test," Nichols told reporters.
The letter and its contents were sent to an Army research facility in Maryland for further analysis and authorities say they expect to know by Tuesday what the suspicious substance is.
"Only time will tell what may or may not develop," said Capitol Hill physician John Eisold.
• Probe Focuses on Trenton, N.J.
Officials said the letter delivered to Daschle's office was postmarked on Sept. 18 in Trenton, N.J. — the same date and place as on the postmark of an envelope holding a brown granular substance containing anthrax spores that was mailed to Brokaw.
The Trenton facility processes mail from 46 other area post offices, making it difficult for federal investigators to pinpoint the precise location from which each letter was mailed.
Postal Inspector Tony Esposito said two employees at the facility were being treated with antibiotics as a precaution.