There is no evidence that the facility has been contaminated or that anyone has been exposed to any health risk, said Thomas Day, vice president of engineering at the Postal Service.
"We believe in the end we're going to have a non-event," said Day.
But he said, "When you have a lab result that is a preliminary positive, you have to take that seriously."
Preliminary field tests for anthrax are often inaccurate.
Day said the Postal Service was taking 30 to 50 samples at the V Street government mail facility, which sorts mail going to the federal government.
The Postal Service said results of the anthrax tests would be announced Wednesday.
Since the anthrax-by-mail attacks in the autumn of 2001, the Fed has been testing its mail in a trailer in a courtyard outside its headquarters in downtown Washington.
Five people died in the October, 2001 attacks, including two postal workers. Eighteen other people were infected by the disease.
The postal processing center that handled the mail used in the 2001 attacks is still closed. Fumigation of the Joseph Curseen, Jr. and Thomas Morris, Jr. Processing and Distribution Center, which processed the letters to Sens. Tom Daschle, D-S.D., and Patrick Leahy, D-Vt, began in mid-December.
The FBI has yet to make an arrest or identify a suspect in the attacks. In December agents revisited the site of one of the attacks, a Florida publishing company.
Anthrax is caused by a naturally-occurring bacteria that can infect the lungs, digestive system or skin. The respiratory strain is often deadly, although it can be treated with powerful antibiotics.