They are not sure whether the discoveries are signs of an attack.
The Fairfax County Fire Department says it decontaminated 42 people who work in a mailroom, sorting letters and packages bound for the Department of Defense, reports CBS News Correspondent Dan Raviv.
As testing continued Tuesday, President Bush was being regularly updated on the situation, White House press secretary Scott McClellan said.
"The initial testing came back positive. There was some additional testing that was done and it was inconclusive," he said. "We're still waiting on more definitive results."
The two pieces of mail, the origins of which were not provided, had been irradiated, so officials believed any anthrax in them was inert when they triggered alarms at the two mail facilities on Monday.
Additional tests and other sensors at the two facilities, one of them at the Pentagon and the other nearby, found no presence of the bacteria, which can be used as a biological weapon. There were no initial reports of illness.
Meanwhile, a hazardous materials team was called Tuesday to a building occupied by the Internal Revenue Service after a report of a powdery substance found in a letter, said Alan Etter, Fire and Emergency Medical Services spokesman. He said no one was evacuated and there were no reports of illness.
Also on Tuesday, a postal facility in Washington that may have handled the Pentagon mail was closed and some 200 workers offered a three-day course of antibiotics as a precaution, said Mayor Anthony Williams. Tests will be conducted at the facility to determine whether any anthrax is present. Officials said none of the workers had reported any unusual health problems.
The Pentagon's mail delivery site, which is separate from the main Pentagon building, was evacuated and shut down Monday after sensors triggered an alarm around 10:30 a.m., spokesman Glenn Flood said. It was expected to remain closed until at least Tuesday while the investigation continued.
It was not clear when sensors at the second Defense Department mailroom were triggered. Pentagon officials said only that a nearby satellite mail facility was closed.
But firefighters in nearby Fairfax County, Va., reported that a military mailroom in the Bailey's Crossroads business district a few miles from the Pentagon had been shut down after a hazardous material was detected, and no one was allowed to leave that building.
Pentagon spokeswoman Lt. Cmdr. Jane Campbell said mail at both facilities had been irradiated before arriving at either one. The radiation treatment should kill any anthrax bacteria, but sensors would still be able to detect it.
She had no information about the origin of the two pieces of mail.
About 175 people work at the Pentagon's mail facility, and an additional 100 may have been in contact with deliveries for the Pentagon, officials said.
Medical personnel took cultures from anyone who may have had contact with those deliveries, and those people were also offered a three-day course of antibiotics and told to watch for the signs of anthrax exposure: fever, sweats and chills.
Follow-up tests were being conducted at the U.S. Army Research Institute for Infectious Diseases at Fort Detrick, Md., officials said. They would take two to three days to complete.
General operations at the Pentagon appeared unaffected.
Anthrax can be spread through the air or by skin contact. Officials noted that sometimes anthrax sensors can give false-positive results.
Several cases involving letters laced with killer substances remain unsolved.
In October 2001, someone sent anthrax in letters through the mail to media and government offices in Washington, Florida and elsewhere, raising fears of bioterrorism. Five people were killed and 17 more sickened.
In October 2003, two letters containing the poison ricin, sent to the Transportation Department and White House, were intercepted before they reached their destination. The letters objected to new rules for long-haul truckers.
A small amount of ricin was discovered Feb. 2, 2004, on a mail-opening machine in the office suite of Senate Majority Leader Bill Frist, R-Tenn. The discovery led to a shutdown of three Senate office buildings for several days, and about two dozen staffers and Capitol police officers underwent decontamination.