The Seattle man, who was not identified, posted threats online earlier today, according to the Seattle Police Department. In a separate posting, the man claimed to have purchased an assault rifle and extra ammunition, police said.
SWAT and hostage negotiators went to the man's home and arrested him shortly before 3:30 p.m. after an incident police described on Twitter as a "standoff."
The man has been previously contacted by law enforcement in the area who are investigating harassment and threats to another mosque, police said.
Seattle police are working with the FBI to investigate the "rapidly developing" case, according to police. Authorities have not announced charges.