Ricin is a highly toxic substance made from castor beans. As little as 500 micrograms, the size of the head of a pin, can kill an adult if inhaled or ingested.
There have been no reports of illness connected to the letters.
FBI agents, Spokane police and U.S. Postal Service inspectors descended on the three-story apartment building Saturday morning and the investigation continued into the afternoon.
FBI spokeswoman Ayn Sandalo Dietrich would not say whether agents were questioning anyone in connection with the case.
"We are not actively looking for a subject," Sandalo Dietrich said. "We are not asking the public's help in bringing someone in."
Despite the hazmat suits, officials said apartment residents were not at risk, and people were seen coming in and out of the brick building in the city's historic Browne's Addition neighborhood.
"There's no public risk," Sandalo Dietrich said.
Sandalo Dietrich would not say specifically why the FBI was searching the apartment.
"Information we developed led us to believe this was a productive spot to search," she said.
Two letters containing the substance were intercepted at the downtown Spokane post office Tuesday.
The Postal Service has received no other reports of similar letters, said Jeremy Leder of the Postal Inspection Service on Saturday.
In a statement following the discovery, the Postal Service said the "crude form of the ricin suggests that it does not present a health risk to U.S. Postal Service personnel or to others who may have come in contact with the letter."
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.