The besieged Postal Service continued spot checks at facilities nationwide Friday as a State Department mail handler lay ill with inhalation anthrax and the nation's bioterror scare widened.

"We still don't know who is responsible," said Homeland Security Director Tom Ridge Thursday at the White House.

The terrorists who sent anthrax to Senate Majority Leader Tom Daschle "intended to use this anthrax as a weapon," he said.

Ridge said the deadly bacteria mailed to Daschle had different characteristics from the anthrax spores that were sent to other locations in the U.S. 

"It is highly concentrated, it is pure, and the spores are smaller," Ridge said. "Therefore, they're more dangerous because they can be more easily absorbed in a person's respiratory system."

Ridge said the type of anthrax used in the U.S. attacks is called Ames. Anthrax researchers have identified Ames, named for the city in Iowa, as the strain used in American bioweapons research. The strain also is used to test vaccines, researchers say.

Three weeks into the nation's unprecedented bioterrorism scare, lawmakers were permitted to return to several of their office buildings on Capitol Hill. And White House spokesman Ari Fleischer said there had been no evidence of anthrax exposure among officials there who came in contact with mail that went through an offsite machine where anthrax was detected earlier in the week.

"We are here to conduct the nation's business. We will not be frightened," said Secretary of State Colin Powell as he appeared before a Senate committee.

But there were words of caution elsewhere. "We are very concerned about additional letters. We would be naive to think this is over yet," said Dr. Julie Gerberding of the Centers for Disease Control and Prevention.

There was further jolting news, a disclosure from officials in New Jersey that a postal worker was being watched for suspected inhalation anthrax, the discovery of two more areas of contamination in a still-closed Senate office building — and then the announcement from the State Department.

Spokesman Richard Boucher said a department employee who works at a mail handling site in Sterling, Va., had become the nation's latest victim of a disease last seen more than two decades ago.

Dr. Ivan Walks, head of Washington's public health department, said the 59-year-old man was hospitalized in guarded condition with inhalation anthrax. Unlike other area residents who have been hit, this patient had been asked whether his job required him to go to the Brentwood postal facility that serves as the main mail processing center for the nation's capital. "His answer was 'never,'" Walks reported.

A second man who works at the same mail facility as the infected worker has flu-like symptoms and is being tested at a hospital, Boucher said Thursday night.

Also, a test anthrax in a mailroom in the Walter Reed Army Institute of Research in Silver Spring, Md., came back positive Thursday, said Charles Dasey, spokesman for U.S. Army Medical Research and Material Command. The institute, which doesn't care for patients, is three miles from the hospital at the Walter Reed Army Medical Center. Dasey said the mailroom at Fort Detrick, Md., which exchanges mail with the institute, also was being tested.

Mail to federal agencies passes through the Brentwood facility, and the latest diagnosis added to the mounting evidence that investigators have not yet found all the anthrax-tainted mail in the area's postal system. Postal Service Vice President Deborah Willhite said the agency would begin testing all government mail intake facilities in the region for signs of anthrax.

In addition, the State Department announced it would test employees at all of its mail annexes and its main facility several blocks from the White House. In all, he said 250 to 300 people are being tested for anthrax exposure, and about 80 people who work at the Sterling facility are receiving the antibiotic Cipro, Boucher said.

"We don't know where it's coming from," Boucher said of the anthrax. Asked about Powell, Boucher said, "We have no reason to suspect he's been exposed."

The announcement brought the number of confirmed anthrax cases to 13 in the past three weeks, including seven cases of inhalation and six of the less dangerous skin form of the disease. Most are linked to anthrax-spiked mail that has passed through New Jersey, New York or the nation's capital.

A bioweapons expert from Sandia National Laboratories said the anthrax-by-mail attacks suggest a degree of sophistication beyond the Al Qaeda terrorist organization.

"Unless they bought it from Iraq or something, it's not likely to be Al Qaeda," Alan Zelicoff said in an interview from Washington, D.C.

What makes the current attacks different from anthrax outbreaks of the past is not the anthrax itself, but rather the way it's been dispersed, Zelicoff said. The anthrax in the current attacks has been treated "with materials that make it float in the air. That's no mean trick; it's a hard thing to do," he said. "It suggests a sophisticated program with a lot of expertise, not in biology ... but in aerosol physics."

One such letter was addressed to Daschle; others were sent to NBC News anchorman Tom Brokaw and to the New York Post. Anthrax has also been found in Florida, where one man died, although authorities have not yet found tainted mail there.

At his White House news conference, Ridge said tests on the anthrax found to date confirmed it is all of the same strain and responds to antibiotics, meaning that "people who are exposed can be treated."

At the same time, he added, the substance in Daschle's mail "has some different characteristics" that make it more easily taken into a victim's lungs. He said that based on the latest lab reports, "it is clear that the terrorists responsible for these attacks intended to use this anthrax as a weapon."

In all, 28 people were exposed to anthrax in and around Daschle's office, and all Senate office buildings were shut down. The discovery of anthrax on a mail machine in a House office building prompted the closure of additional office buildings.

Daschle said one wing in the building that houses his office would remain sealed off for the indefinite future. "I am very confident that we will be able to seal it in a way that will provide us complete confidence that we can access the rest of the building without any hazardous exposure," he told reporters in the Capitol.

Officials learned Wednesday of additional anthrax in the same building when tests taken from a freight elevator turned out positive. Congressional sources, speaking on condition of anonymity, said the elevator is used routinely to convey express mail to the Hart Building, connected by underground tunnel to the Senate's main mail room in the building next door.

Thus far, an estimated 10,000 people have been given antibiotics as a precaution against contracting anthrax, many of them postal workers in New York, New Jersey and the nation's capital. And in recent days, the Postal Service and Bush administration have launched an intensive effort to assure the safety of the mail system.

Ridge said that as of midday, officials had begun environmental testing at 200 postal facilities along the Eastern corridor, presumably the region from New York to Washington.

"I want to reiterate, there is no indication of any new exposure at this time at these sites," he said.

The Associated Press contributed to this report.