After several days of fumigating and more than a week of testing, health officials have not been able to wipe out all the anthrax spores found in Hart Senate office building, U.S. Capitol Police reported Friday.

Officials said they would go into the building again to try to kill the lingering spores.

Environmental Protection Agency technicians planned to begin the new fumigation — once again using chlorine dioxide gas — at the Hart Senate Office Building later in the day.

"We're in the home stretch. We have found minimal contamination. This is just sort of a belt-and-suspenders approach. We want to make sure that it's very safe when people go back in," said Richard Rupert, the EPA's on-site coordinator.

Hart building was closed Oct. 17, two days after an anthrax-filled envelope was received in the personal offices of Senate Majority Leader Tom Daschle, D-S.D.

EPA officials ordered the new fumigation after finding anthrax on nine of 380 sterile gauze wipe samples taken from Daschle's office suite.

"We had less than 10 that came back positive and those were all pretty much in one area that was always one of the hottest areas," Rupert said.

Officials are still awaiting the results of 2,994 test strips that have been sent to university and Army labs for evaluation. They said those were more difficult to assess because the strips had never before been used with the gas.

Rupert said the affected area was the cold-air return in the ventilation system of Daschle's office.

The anthrax in the Daschle letter has been described as particularly potent and refined to be airborne. A similar letter was sent to Sen. Patrick Leahy, D-Vt., chairman of the Senate Judiciary Committee, but it never reached its destination, having been collected in a quarantined batch of mail before it could be opened.

Investigators analyzing the Leahy letter say its spores appear identical to those in the Daschle letter.

News of the continued contamination may come as a surprise to Daschle, who prior to the announcement said he knew the ventilation system was to be remediated this weekend but he had every reason to believe that his offices would be re-opened on schedule, which has slid back to early January. He also said he had no reason to expect that more gas would be introduced in his office.

Daschle did say he has harbored some doubts that the building would ever be safe to go back inside but added that the anthrax attack was "the single, largest biological attack in our nation's history."

"We're plowing a lot of new ground. There are a lot of pioneering efforts under way here. And so we have to be realistic and we have to be patient, even though it's very difficult to be either, given the tremendous uncertainty, as well as the many practical difficulties members and their staffs have had to endure."

"Once it's safe, that's when it will be reopened," said Lt. Dan Nichols, spokesman for the Capitol Police.

The Associated Press contributed to this report.