A letter sent to the office of Senate Majority Leader Tom Daschle, D-S.D., has tested positive for anthrax.

The letter arrived at about 10:30 a.m. Monday. When it was opened, a white, powdery substance fell out of it. Capitol Police were called, the office was sealed and workers were given tests for anthrax exposure, according to Capitol Police Lieutenant Dan Nichols.

Nichols said police tested the substance, which showed up as anthrax.

"The first field test came back positive for anthrax. In order to confirm that, we conducted a second field test and that field test also came back positive for anthrax," Nichols said.

The letter containing the substance was sent to the U.S. Army facility at Fort Dietrich, Md., for additional testing.

The office is quarantined and officially closed, and about 40 staff members are being tested for exposure and decontaminated, Daschle told reporters Monday. Pizza was delivered to those who are expected to be stuck there for a while, according to a Fox News producer on the scene. Capitol Police are moving in and out of the scene.

Senate physician John F. Eisold said the medical response is similar to the responses to other anthrax cases that have occurred around the country in the last week.

"Appropriate people were identified who potentially could have come in contact with the exposure. They have been swabbed and they will be tested now to see if they do indeed have any of the spores," Eisold said, adding that in the meantime they will be treated with the antibiotic Cipro.

Daschle said he was "very, very disappointed and angered" by the latest event.

"I am concerned deeply for my staff and I feel so badly for each of them. They are innocent people caught up in a matter for which they have nothing to do," Daschle said.

The majority leader keeps two offices in the Senate. The letter was sent to Daschle's personal office, which handles business for his constituents. The second office is for his role as majority leader, which is dedicated to Senate business. 

Daschle's personal office is located on the fifth floor of the Hart Senate Office Building. Both that floor and the sixth floor were sealed off. 

Daschle said that he did not speak directly to the staffer involved in the exposure because that person was being treated. However, he had spoken to the rest of his staff, family members and other leaders in Congress.

The letter to Daschle was postmarked Sept. 18 from Trenton, N.J., said postal inspector Tony Esposito. A letter containing anthrax mailed to NBC anchor Tom Brokaw also was postmarked Trenton. The letter never reached Daschle directly. 

President Bush said the letter "had been wrapped a lot." 

Within a few hours of the letter's delivery to Daschle's office, officials in the House and Senate issued orders sequestering all mail for screening.

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."

A letter has also been sent to all Senate offices explaining what safety measures offices should take if they are exposed to anthrax.

Nichols said that he could not speak to specifics of the criminal investigation, but Capitol Police were prepared for such an event.

"We are very limited in the information that we can put forward. This is not totally unexpected, though. U.S. Capitol Police have been working closely with leadership and other law enforcement agencies and also the Department of Defense in preparation for this type of event," Nichols said.

He said that staff and members of Congress had been educated on what to do and responded accordingly. Daschle's staff followed protocols by notifying the police and Capitol physician, Nichols said.

The latest case of anthrax exposure comes after days of unsettling reports of anthrax scares in Florida, New York and Nevada, including the death of one man in Florida last week.

Speaking to reporters at the White House alongside Italian Prime Minister Silvio Berlusconi, the president said "there may be some possible link" between Usama bin Laden and the recent flurry of anthrax-related mailings.

"I wouldn't put it past him but we don't have any hard evidence," Bush said of the man suspected as the leader behind Sept. 11 terrorist attacks in New York and Washington.

"The key thing for the American people is to be cautious," Bush said.

The Associated Press contributed to this report.