White House officials have just about conceded that the anthrax spores sent through the postal service to U.S. government agencies and media outlets were produced in the United States. 

Press secretary Ari Fleischer said the evidence is not conclusive, but it is increasingly "looking like it was a domestic source." 

Fleischer said even if the anthrax appears to be American-made, its origin and the sender were unlikely to be related.

"There's a big difference between the source of it and who sent it, because the two do not have to be tied," he said. 

On Monday, Army officials rejected a newspaper report over the weekend that suggested the genetic makeup of the anthrax used in letters sent to Senate Majority Leader Tom Daschle, D-S.D. and Senate Judiciary Committee Chairman Patrick Leahy, D-Vt. indicates it originated from the Army's stockpile.

The Washington Post reported that the anthrax spores that have shut down the Hart Senate Office Building indefinitely match those from the U.S. Army Medical Research Institute of Infectious Disease.

However, Army spokesman Chuck Dasey said the genetic makeup does not indicate anything about the source of the anthrax or who sent it. Dasey said the Army's anthrax came from the Department of Agriculture, and it was shared with five other labs around the United States, Canada and Britain.

"You can't say it all came from USAMRIID. We got it from another lab in the first place, and so presumably USAMRIID is not the only lab that got it from the Department of Agriculture," Dasey said.

Investigators still do not know who sent the bacteria that showed up in New York, Connecticut, Washington, D.C., Florida, New Jersey and in diplomatic pouches overseas.

Hart Still Contaminated

In the meantime, the second round of teams trying to clean up the last anthrax at the Hart Building has failed. Crews were not able to pump enough chlorine dioxide gas into the building on Sunday during a six-hour process that focused on the ventilation system.

U.S. Capitol Police spokesman Lt. Dan Nichols said a mechanical problem with the machines that pump the gas prevented enough from entering the HVAC system. 

Crews are trying to clean the HVAC, plus 10 to 12 suites.  They also want to do a room-by-room check before opening the building.

"Clearly, the building is not ready to be re-occupied," Nichols said. 

The second attempt at fumigating the building was aimed at killing the last remaining traces of anthrax that were first discovered in Daschle's offices in October, when an anthrax-filled letter was opened by an aide to the top Democrat. Clean-up crews from the Environmental Protection Agency are using the liquid form of chlorine dioxide for Daschle's office.

In the Longworth House Office Building, offices belonging to four representatives have been cleaned but remain closed. Nichols said the offices are being tested for remaining spores.

"The remediation process has been completed. We're in the renovation process now to go in and clean up to make sure that the offices are habitable, and we hope to have that concluded this week and have the workers back into those four offices at Longworth," Nichols said.

The closures are not just an inconvenience for workers on Capitol Hill. It has also been a difficult job for clean-up crews, who are dealing with a bacteria that has been finely milled and which spreads extremely easily.

"The crews are tired. A lot of people have been up with occasional catnaps in the back of a trailer or in their car. So, they're overworked. They're tired. But we're really upbeat we're finally at a point we can finally pick up the fumigation," said Richard Rupert, the EPA's on-site coordinator.

The U.S. Postal Service is waiting until decontamination is complete at the Hart building and environmental tests show it's clear of anthrax before the postal service decides what to do about the Brentwood Postal Facility in Washington, D.C.

Two workers at the Brentwood post office died from inhalation anthrax. Officials believe Joseph Curseen, Jr., and Thomas Morris, Jr., came in contact with the anthrax-laced letter mailed to Daschle, D-S.D. The Brentwood facility has been closed since October. All mail that was left in the post office was taken to facility in New Jersey where it was irradiated.

As the clean-up process continues, federal officials are considering whether to recommend the anthrax vaccine to people in high-risk groups, who had exposure to contaminated letters. The vaccine would be voluntary, and could potentially affect up to 3,000 people who had high levels of exposure in New Jersey, where the letters were originally postmarked, as well as in New York, Florida, Maryland and Washington, D.C..

Secretary of Health and Human Services Tommy Thompson could make a decision on the vaccine this week. The other option includes handing out more antibiotics, another 30 days on top of the basic 60-day treatment affected workers have already had.

The Associated Press contributed to this report.

Catherine Herridge is an award-winning Chief Intelligence correspondent for FOX News Channel (FNC) based in Washington, D.C. She covers intelligence, the Justice Department and the Department of Homeland Security. Herridge joined FNC in 1996 as a London-based correspondent.