Secret Service: Powder, threat found at McCain's Denver office

DENVER, Colorado (CNN) - An envelope containing an unidentified powder and a threat against Senator John McCain was found at the presidential hopeful's Denver office, a U.S. Secret Service spokesman said Thursday.

"Agents and officers are responding," said spokesman Malcolm Wiley.

The letter was received at 3 p.m. MT (5 p.m. ET) at McCain's regional campaign office in the suburbs outside Denver, McCain's campaign said.

Authorities told CNN that the substance is being field tested and the situation is under control. A threatening letter was in the envelope along with the powder, authorities said.