The FBI's announcement came after an extensive search of 42-year-old Ryan Kelly Chamberlain II's apartment at the corner of Jackson and Polk streets in the city's Russian Hill neighborhood, according to KTVU-TV Channel 2.
Investigators in protective suits searched Chamberlain's apartment for several hours Saturday. A local TV station reported that agents found a large stash of what appeared to be the deadly poison ricin, but FBI spokesman Peter Lee said Sunday authorities do not believe the white substance was ricin.
The apartment complex was not evacuated during the search, and Lee said there was "no threat to public safety."
The FBI described Chamberlin as white, six-foot-three and weighing about 225 pounds, with blue eyes and brown hair.
Authorities said Chamberlain was last seen driving a white 2008 Nissan Altima with either with either California or Texas license plates 7FQY085 and BX9M042, respectively.
Building owner Martin Harband told the station that Chamberlain has rented an apartment in the complex for the last two years. He said Chamberlain "seemed normal," never showing signs of unusual behavior.