The letters were postmarked last Tuesday in Spokane and addressed to the downtown post office and the adjacent federal building.
Few details have been released, but investigators in hazmat suits spent from 7 a.m. to 10:30 p.m. Saturday executing a search warrant at a three-story apartment building. Authorities said there was no public health risk.
Agency spokeswoman Ayn Sandalo Dietrich said Sunday investigators appreciated the patience of the building's residents and neighbors.
The Spokane investigation comes a month after letters containing ricin were addressed to President Barack Obama, a U.S. senator and a Mississippi judge. A Mississippi man has been arrested in that case.