A threatening letter containing an unidentified white powder was sent to a Republican presidential contender John McCain campaign office in the Denver suburb of Centennial, according to authorities. A hazardous materials team was trying to determine if it was harmful. Andy Lyon of Parker South Metro Fire Rescue Authority said a worker at the office discovered the power after opening the letter Thursday afternoon. Lyon added that no injuries had been reported, but some people may have gone to hospitals on their own. Postal Inspector Jo Jan Henderson said agents from her office were at the scene. Officials with the FBI, Secret Service, Arapahoe County Sheriff's Department and McCain's Colorado campaign did not immediately return calls. The threat came four days before the Democratic National Convention opens in Denver.