The FBI is investigating a package containing toxic powder and Nazi insignias that was sent to the Tucson, Ariz., office of Rep. Raul Grijalva (D) on Thursday.
CNN reported that an envelope containing a white, powdery substance and two swastikas was opened by a staffer. The Tucson Fire Department confirmed that the substance was toxic.
Nearly a dozen people were inside the office when the envelope was opened, spokesman Adam Sarvana told CNN. Everyone in the office was checked by local authorities and none required further treatment.
The offices of Grijalva, who faces a competitive reelection on Nov. 2, had already been targeted twice this year.
In April, both of his district offices were forced to close after staff members received death threats following the passage of his state's controversial immigration law.
Grijalva, the chairman of the Congressional Progressive Caucus, is a staunch opponent of the law and had called for a boycott of Arizona as a protest measure.
Then in July, his Yuma, Ariz., office was forced to close after a bullet shattered a window inside.