McCain Office Is Sent White Powder, Threat

John McCain's campaign office in Denver, Colorado, has received a letter containing a threat and "an amount of white powder in it," a McCain campaign spokesman told CBS News' Michelle Levi.

"We immediately notified local and federal law enforcement agencies and are looking to cooperate with them," McCain spokesman Jeff Sadosky said.

The city of Denver is at a heightened security level, as the Secret Service and other officials prepare for the Democratic National Convention, which is set to begin on Monday.

The Arapaho County Police Station was abuzz with activity when they were called to confirm receiving word of the threat, Levi reports. They have started a call line as officials return media calls one by one.