Capitol Police Lt. Dan Nichols said the letter, which was opened by Daschle aides, contained a powdery substance. He said two preliminary field tests on the letter were positive for anthrax. The letter was then sent to an Army medical research facility at Fort Detrick, Md., for further tests.
``There was an exposure when the letter was opened,'' Nichols said.
People who were exposed were being treated with Cipro, an antibiotic, said a Capitol physician. There was no immediate indication whether any of those exposed had anthrax spores in their bodies.
Daschle said his office in the Hart building a block from the Capitol had been quarantined and closed. Emergency medical vehicles were parked outside the building.
He said there were 40 people in his office at the time, but that he doesn't know how many of them may have come in contact with the letter. He said he also was gratified that the response was so quick.
``We have to be alert, we have to recognize that the risk is higher than it was a couple of weeks ago but we have to live our lives,'' he said.
Speaking to reporters at the White House, Bush said ``there may be some possible link'' between Osama bin Laden and a recent flurry of anthrax-related developments.
``I wouldn't put it past him but we don't have any hard evidence,'' he said of the man suspected as the leader behind Sept. 11 terrorist attacks in New York and Washington that killed thousands.
Within a few hours of the delivery of the letter to Daschle's office, officials in the House and Senate issued orders to all congressional offices to refrain from opening mail.
A memo from the House sergeant-at-arms said the mail would be ``picked up ... for additional screening and returned to you as soon as possible.''
Bush's national security adviser, Condoleezza Rice, said precautions were being taken at the White House with regard to mail, but added she was not aware of any tainted letters being delivered there. She would not provide details on the security measures.
``Like everybody else, we are being very cautious about what we open,'' Rice said.
The suspicious package was received at the majority leader's office in a Senate office building across the street from the Capitol.
Separately, one source said that when it was opened, a powdery white substance fell out. Capitol Police were summoned, the office sealed, and the workers immediately given a test for anthrax exposure. There was no immediate word on the results of those tests.
But Bush, in responding to a reporter's question, said he had just talked with Daschle. ``His office received a letter and it had anthrax in it. The letter was field-tested. And the staffers that have been exposed are being treated.''
The president made his comments after a meeting with Italian Prime Minister Silvio Berlusconi, the latest in a steady stream of foreign leaders to visit Washington in the wake of the terrorist attacks.
The president said additional tests are being conducted on the letter. It ``had been wrapped a lot,'' he said, and there was ``powder within the confines of the envelope.''
He said the powder itself had been sent to the Centers for Disease Control and Prevention for additional testing.
The disclosure came after days of unsettling reports of anthrax scares in three states, including the death of one man in Florida last week.
``The key thing for the American people is to be cautious,'' said Bush.