A mail check at Grijalva's local office Thursday uncovered a white powdery substance and drawings of two swastikas inside an envelope, Adam Sarvana told CNN.
The Tucson Fire Department confirmed the substance was toxic, Sarvana said. Nearly a dozen people who were at the office when the substance was found were examined and sent home.
Scientists at an FBI laboratory in Phoenix were conducting a full analysis of the powder, Sarvana said.
The incident is the third security issue at Grijalva's district offices this year, the spokesman told CNN. Two offices were temporarily closed in April after receiving threats about Grijalva's position on immigration legislation. In July, a bullet shattered a window in his Yuma office, KVOA-TV, Tucson, reported.
"I do feel a great deal of concern for the people that work with me, for me. I feel concerned for the constituents that come to that office," Grijalva told KVOA. "Obviously, we're going to have to make more changes."