Spokane police officers, FBI agents and U.S. Postal Inspection Service officials descended on the apartment Saturday morning.
No arrests have been made, and an FBI spokesperson did not say whether agents are questioning anyone in connection with the case.
The motive for the poisoned letters remains unknown, and there have been no reports of illness connected to the letters.
Despite the hazmat suits, officials say apartment residents are not at risk, and people were seen coming in and out of the building.
Ricin is a highly toxic substance. Tiny amounts can be deadly if inhaled or ingested.
Two letters containing the substance were intercepted at the downtown Spokane post office Tuesday. There have been no reports of illness connected to the letters.
The Associated Press contributed to this report.