Republican presidential candidate Sen. John McCain, R-Ariz.,talks about various issues, including immigration and border security, Wednesday, Aug. 20, 2008 during a town hall meeting at New Mexico State University in Las Cruces, N.M. (AP Photo/The La Cruces Sun-News, Shari Vialpando)
Authorities say a threatening letter containing an unidentified white powder was sent to a John McCain campaign office in the Denver suburb of Centennial.