The anthrax that killed five people appears to have been produced in the United States, the White House said Monday, but investigators still don't know who mailed it. On Capitol Hill, a second attempt to sanitize a contaminated Senate office building failed.

White House press secretary Ari Fleischer said the evidence on the anthrax sent to Sens. Tom Daschle, D-S.D., and Patrick Leahy, D-Vt., is increasingly "looking like it was a domestic source."

President Bush emphasized that the case remains unsolved.

"We're still looking," he said. "We've all got different feelings about it. We're gathering as much information [as possible]." The president promised that as soon as some conclusions are reached "we'll share it with the American people."

At the State Department, the FBI was called to examine a white powder found in an envelope addressed to Deputy Secretary of State Richard Armitage. The envelope moved through the regular U.S. mail system and officials assumed that it had been irradiated, said Lynn Cassel, a department spokeswoman.

Cassel said the letter was addressed in "block letters," but she did not know if the envelope or the writing resembled that found on previous, anthrax-tainted letters.

On Capitol Hill, a military physician told members of Daschle's staff at a briefing that he would recommend they take an anthrax vaccine, which is considered experimental, if Health and Human Services Tommy Thompson approves its use. Thompson was expected to make his decision this week.

Technicians, meanwhile, ran into more problems trying to sanitize a Senate office building contaminated by lingering anthrax spores from the letter sent to Daschle in mid-October.

Attempts to pump poisonous gas into the Hart Senate Office Building's ventilation system to kill any remaining spores failed early Monday, said Capitol Police spokesman Lt. Dan Nichols.

He said a mechanical problem kept the gas from reaching the saturation point needed to kill the anthrax. Technicians worked from 9 p.m Sunday to 3 a.m. Monday before they abandoned the effort.

"The goal was to get 500 parts per million. We did not achieve that goal," said Nichols. He said he did not know what caused the problem, which did not show up the first time they used the gas on the building.

Nichols said he was not aware of when another attempt would be made.

The Hart building has remained closed since Oct. 17, two days after an anthrax-filled letter was received in Daschle's office. The EPA reported Friday that traces of anthrax remained after the initial fumigation.

In New Mexico, officials at the Los Alamos National Laboratory, who have been conducting research in the anthrax investigation, acknowledged that they received a sample of viable, or living, anthrax, despite telling area residents that its research was limited to inactive spores.

Los Alamos, which has been analyzing the genetic content of the anthrax used in the attacks, thought it was receiving dead spores from Northern Arizona University. But after the paperwork was filled out, the university discovered that the spores were alive and able to grow.

An internal Los Alamos report indicated that its lab was not certified to handle live anthrax, prompting concern from a member of Congress, Rep. Edward Markey, D-Mass. In fact, federal officials said Monday, the internal report was wrong and Los Alamos is certified to handle the live bacteria.

The Centers for Disease Control and Prevention has confirmed 18 cases of anthrax infection nationwide — 11 cases of inhalation anthrax and seven through the skin — since the anthrax-by-mail attacks began in October. Five people have died, all from inhalation anthrax.