Sen. Roger Wicker is acknowledging a letter sent to his Washington office is the subject of an investigation by the United States Capitol Police and the FBI.

Wicker says there is an “ongoing investigation” and that he wants to thank law enforcement officials for keeping the Capitol complex safe. An envelope addressed to Wicker’s office on Tuesday twice tested positive for Ricin.

Wicker says he and his wife, Gayle, appreciate “everyone’s thoughts and prayers.”