(CBS/ AP) Threatening letters containing an unidentified white powder have been received at John McCain's campaign offices in Denver, Colorado, and Manchester, New Hampshire, CBS News has learned.
McCain's campaign office in Denver 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," spokesman Jeff Sadosky said.
A hazardous materials team was trying to determine if the powder found in Colorado is harmful.
Another McCain spokesperson later said that McCain's New Hampshire office "received a similar letter."
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.
A staffer in Denver opened the envelope containing the white substance, and the building was evacuated, reports CBS4 in Denver. The FBI, Secret Service, and local authorities responded to the scene.
As many as twelve people were exposed to the substance, CBS4 reports, but no one apparently reported suffering any symptoms. Seven people drove themselves to Sky Ridge Medical Center, but none showed any symptoms of exposure to a toxic substance, said hospital spokeswoman Linda Watson.