This was the second time workers, led by the Environmental Protection Agency, had tried to use the gas to clear the building of anthrax spores since it was contaminated Oct. 15.
Nichols said it was unclear when workers would try again or how long the building would be quarantined.
"This is a situation that has never been dealt with before," Nichols said. "We are going to overcome the problems that developed. In the near future, at a time yet to be determined, we are going to go back one more time and introduce chlorine dioxide gas into the ventilation system."
EPA officials earlier Monday said workers had finished fumigating the narrow section of the Hart Senate Office Building at 3 a.m., EPA spokeswoman Bonnie Piper said.
Nichols said any confusion was due to "semantics."
The effort to address anthrax spores with the chlorine dioxide gas covered only the ventilation systems in the building on floors one through nine in the Southeast corner. That section includes the office suite for Senate Majority Leader Tom Daschle, D-S.D., on floors five and six that was contaminated on Oct. 15 when staff opened an anthrax-laden letter.
Workers are using other methods to continue decontaminating individual offices in the building.
The fumigation work that started Sunday night and ended early Monday was the second round in the Hart building in as many weeks. That work was planned for Friday night, but delayed because the humidity in the building was not at the correct level. The method calls for a 75 percent humidity level.
The decontamination efforts took on added importance over the weekend when federal health officials suggested workers who were exposed to a large number of spores, such as those in Daschle's office and U.S. Postal Services workers who handled the letter, might still be at risk for inhalation anthrax infection even after taking the recommended 60-day regimen of antibiotics.
The Centers for Disease Control and Prevention in Atlanta this week is expected to recommend to Health and Human Services Secretary Tommy Thompson that Daschle staffers and other at higher risk be allowed to participate in a special program that would make the anthrax vaccine available to them, in three doses, as well as an additional 30-day supply of antibiotics.
Studies on animals have shown even after taking antibiotics for 60 days, some animals develop anthrax infection and die when the medication is stopped, indicating the lungs may retain the spores for longer than 60 days after exposure.
Other studies found only animals given the anthrax vaccine and antibiotics were fully protected from lingering spores and any potential new exposure.
Two letters containing a powder containing anthrax spores were delivered to Capitol Hill and two others went to media in Florida and New York.
Genetic tests indicate the anthrax delivered to Capitol Hill was identical to that kept by the U.S. Army for more than two decades, The Washington Post reported Sunday.
Scientists familiar with the most recent tests told the paper that many labs have the Ames strain of anthrax involved in the recent attacks, but only five have spores with perfect genetic matches to those in the Senate letters.
All those labs can trace back their samples to a single U.S. military source: the U.S. Army Medical Research Institute of Infectious Disease at Fort Detrick, Md., the paper said.
Those matching samples are at Fort Detrick, the Dugway Proving Ground military research facility in Utah, a British military lab called Porton Down, and microbial depositories at Louisiana State University and Northern Arizona University in Flagstaff.
The Post said the FBI's investigation into the anthrax attacks was focusing on whether U.S. government bioweapons research programs may have been the source of anthrax used in the attacks.
The focus also is on a contractor that worked with the CIA.
There have been 22 cases of confirmed and suspected anthrax infection, including five deaths from inhalation anthrax, the most serious form of the infection.
An envelope containing a suspicious white powder was opened Monday in the suite of offices belonging to Deputy Secretary of State Richard Armitage.
"This afternoon an envelope containing white powder was opened in the deputy secretary's office suite," Eliza Koch, a State Department spokeswoman, said.
"The envelope was addressed to the deputy secretary and arrived through the US postal system. Because it came through the U.S. postal system we assume it was irradiated and therefore poses no immediate health threat."
A State Department official said the envelope addressed had a return address from Texas and contained only white powder with no letter.
"We don't know if it's a hoax," a senior official said.
Government laboratories examined nearly 70,000 suspected environmental samples all over the Unites States during the current anthrax
bioterrorism attack, but about 45,000 of those samples were not from the scenes -- South Florida, Connecticut and the New York and Washington
metropolitan areas -- where there really was anthrax.
Stephen Morse, associate director for bioterrorism preparedness and response at the Centers for Disease Control and Prevention's National Center for Infectious Diseases, Atlanta, Ga., explained how the Laboratory Response Network, established by the CDC and other agencies prior to Sept. 11, handled the specimens following the anthrax attacks.
During a symposium on bioterrorism in Chicago, Ill., at the annual Interscience Conference on Antimicrobial Agents and Chemotherapy, sponsored
by the American Society for Microbiology, Morse said 7,000 samples were examined by laboratories of the CDC; another 9,500 samples were scrutinized
at the United States Army Medical Research Institute of Infectious Diseases in Fort Detrick, Md.
"Among those states that actually had anthrax, around 7,000 specimens went to those states' laboratories," he said Monday, "but around
45,000 specimens went to the states that had no anthrax. A lot of these specimens were hoax letters and all sorts of powders in envelopes that
someone was worried about.
"All in all close to 70,000 specimens were processed by members of the laboratory Response Network since Oct. 4," the date that photo editor
Bob Stevens was diagnosed with inhalation anthrax infection in Lantana, Fla. Since then, four other people have died.
(Ed Susman and State Department Correspondent Eli Lake contributed to this report)