The poisonous gas pumped into the Hart Senate Office Building over the weekend appears to have killed any remaining anthrax spores, Environmental Protection Agency officials said Tuesday.

The agency must await lab tests to confirm the preliminary results, but monitoring equipment suggested the fumigation worked on the third try. Earlier attempts failed when officials could not create the necessary levels of humidity.

"We feel very comfortable that we had a very successful fumigation," Richard Rupert, the EPA's on-scene coordinator said in an interview.

"We feel that based on the humidity numbers, the temperature and the concentration of chlorine dioxide, we were able to effectively destroy any anthrax spores."

Technicians under the environmental agency's direction began pumping chlorine dioxide gas into the heating and ventilation system of the southeast quadrant of the building at 9 a.m. Friday. The process was completed at 4:30 a.m. Monday.

The quadrant had tested positive for anthrax spores after a fumigation effort on Dec. 1.  A second attempt was thwarted on Dec. 16 when technicians failed to achieve the necessary levels of humidity for the gas to work properly.

The building, where half the 100 senators have their offices, closed Oct. 17 two days after an aide to Senate Majority Leader Tom Daschle opened an anthrax-tainted letter.

Daschle said Sunday there was a reasonable possibility the building would reopen this month.

Officials are waiting for results, possibly later this week, on two types of laboratory tests.

One analysis is under way on more than 400 Band-Aid-size strips that will provide the results of the fumigation. Separate tests are checking samples taken from swabbing and vacuuming surfaces.

The test strips contain bacteria more resistant to the gas fumigation than anthrax. If that bacteria is found dead, it will suggest any remaining anthrax is too.

The surface samples are gathered by: small vacuum cleaners that have sock-like filters that are then analyzed; swabbing surfaces with something similar to a Q-Tip; and by wiping surfaces with a synthetic cloth the size of a handkerchief.

Rupert said more than 4,000 samples were taken from every room in the Hart building, which has offices surrounding a large open space in the center.

Meanwhile, with a Jan. 7 deadline for inoculations approaching, fewer than 100 people have opted to receive an anthrax vaccine out of thousands eligible, according to the Centers for Disease Control and Prevention.

Of the more than 3,000 people who have sought counseling to help decide if they need the extra therapy, more than 700 chose to take additional antibiotics.

The CDC offered voluntary extra therapy -- either 40 days of additional antibiotics or vaccine -- to thousands of people exposed to anthrax in the attacks. The reason: animal studies suggest that in rare cases, anthrax can lurk in the body for more than the 60 days these people were prescribed antibiotics, erupting after the drug treatment is finished.

Four dozen Capitol Hill workers received the first vaccinations on Dec. 20. Since then, an additional 39 people, mostly postal workers, in Washington, New York, New Jersey and Florida have been vaccinated, CDC figures show. U.S. Postal Service officials counted an additional 10 inoculations.