“Initial tests indicated the possible presence of a biotoxin,” said Heather Rosenker, spokeswoman for the Department of Homeland Security in Washington DC, which was monitoring the incident.
A Homeland Security official later declared that comprehensive testing showed no signs of any biotoxins in the powder.
Seattle TV station KING had raised concerns after it reported the powder, which spilled from two white envelopes, possibly contained biotoxins that could cause botulism or plague.
About 94 people were evacuated from the mail distribution centre, four of whom were decontaminated and placed in hospital for observation.
An army unit was brought in to investigate and carry out further tests.
Jeff Scobba, a postal inspector in Tacoma, told MSNBC that a preliminary test ruled out the possibility that the substance was anthrax but declined to comment on fears that the powder contained biotoxins.
In the weeks following the September 11 attacks, letters containing anthrax spores were mailed through a New Jersey postal centre to news media offices in Washington, New York and Florida, resulting in five deaths.
Anthrax-laced letters were also sent to Senate Minority Leader Tom Daschle and Vermont Senator Patrick Leahy, both Democrats.
No arrests have been made despite a $2.5 million reward. Law enforcement agencies said they suspected the mailings were the work of a disaffected individual, and not of a terrorist network like al-Qaida.