An extended four-day effort to kill trace amounts of anthrax spores from part of the closed Hart Senate Office Building was brought to an end Monday. Officials said they hope it is the last time they have to pour poisonous gas into the building that normally houses the offices of half the Senate's 100 members.

"We let it run longer for a measure of confidence," Environmental Protection Agency spokeswoman Bonnie Piper said. "It's the current plan not to use chlorine dioxide gas again."

Liquid and foam decontaminants and special particle-filtering vacuums are still being used in 11 senators' offices in the building.

Technicians overseen by the EPA officials on Friday began filling portions of the building's heating and ventilation system with the gas. They used steam to elevate the humidity to 75 percent, the threshold at which the gas will adhere to any lingering spores.

The new fumigation had been expected to take up to 24 hours. An earlier effort two weeks ago failed due to equipment problems. After the gas was spread, a second chemical, sodium bisulfite, was used to break down the gas, which also quickly dissipates. The EPA monitored the air outside and reported no dangerous releases.

The building has been closed since Oct. 17, two days after an aide to Senate Majority Leader Tom Daschle opened an anthrax-tainted letter. The EPA used chlorine dioxide gas to kill anthrax spores for the first time in early December, spreading it in Daschle's suite of offices on two floors.

Over the past weekend, the gas was pumped into the air ducts serving the southeast quadrant of the building, an area that officials had said earlier would not need to be fumigated. As a precaution, the EPA also had closed the Dirksen Senate Office Building, whose underground corridors connect with the Hart building. The Dirksen building reopened Monday.

Workers Monday were still removing test strips used to test the effectiveness of the gas from the Hart building. The strips contain bacteria more resistant to the chlorine dioxide. If the gas kills those bacteria, it suggests the anthrax spores also are dead.

EPA officials had no word on when the Hart building might reopen, though they previously expressed hope that might occur in early January.

"There is a reasonable possibility that we'll see that the building open sometime in the early part of next month, perhaps as early as next week," Daschle, D-S.D., said Sunday on NBC's "Meet the Press."

After consulting with the federal Centers for Disease Control and Prevention and the attending physician of the Capitol, the EPA will turn the Hart building over to the architect of the Capitol for a decision on when it should reopen, Piper said.

Lt. Dan Nichols, spokesman for the U.S. Capitol Police, said Sunday the work would not be rushed. "This is the site of the largest bioterrorism attack in the United States," he said. "It's reasonable to be patient."