TACOMA, Washington (CNN) -- A mail distribution center in Tacoma, Washington, was evacuated early Tuesday after a powdery brown substance initially tested positive for the toxins that cause botulism and plague, federal officials said.
The officials emphasized that initial field tests often prove to be inaccurate.
A fire official on the scene said, "We don't have any firm results of that," when asked about the reports of botulism or plague toxins.
The powder was found in two envelopes, one addressed to the local police department and the other to the U.S. State Department, said Gerry McKiernan, a spokesman for the U.S. Postal Service.
Maj. Brad Jackson of the National Guard described the substance found in the envelopes as "a very suspicious brown powder... "A number of people were exposed and are being treated."
All 93 employees in the downtown building were sent home, officials said. Four of the workers were decontaminated at the building, taken to St. Joseph's Hospital for checkups and quarantined.
The Department of Homeland Security in Washington confirmed initial field tests for a biotoxin substance were positive, but also warned false positives are common, and that additional testing was needed.
The U.S. Postal Service said the initial tests were negative for anthrax, a deadly substance that contaminated postal facilities on the East Coast during the nation's post-September 11 security shakeout.
McKiernan said the two letters were discovered in the postage-due area of the facility at 1:25 a.m. and had powder in them.
The Tacoma Fire Department responded to the call, evacuated the building and did some preliminary tests, which showed the powder may be a biotoxin.
The Fire Department then called in the Civil Support Team based out of Camp Murray, which Jon Lendosky, assistant chief of the city's Emergency Medical Services, described as "the military version of a hazmat [hazardous materials] team."
A mobile laboratory from the Washington National Guard's 10th Civil Support Team was conducting further tests on the substance, said Ken Parrish of the state's Emergency Management Division.
The substance was being taken to the Washington State Health Lab in Shoreline for definitive analysis.
Lendosky said it was not known how long the envelopes had been in the facility.
