A suspicious letter containing an unidentified powder reportedly arrived at U.S. Rep. Al Green’s Houston office. It was addressed to the congressman, who said he would not be intimidated.

Green showed an image of the envelope at a news conference. It bore a San Antonio postmark dated Oct. 13. The FBI, Houston police and fire departments, and U.S. Capitol Police responded and are investigating. Authorities have not publicly identified the substance or announced who sent the letter.

Green said he was concerned about the safety of congressional staff and constituents who come to his office for help. The incident comes amid a nationwide investigation into threatening mail.