Emergency personnel outside the Colorado headquarters of Republican presidential candidate Sen. John McCain, on Thursday. (David Zalubowski/Associated Press)

A threatening letter containing an unidentified white powder was sent to a John McCain campaign office in Colorado on Thursday, causing about 40 people to evacuate the building, officials said.

No injuries were immediately reported.

At least 19 people were examined at hospitals or were quarantined outside the office in Centennial, Colo., a Denver suburb, while authorities tried to determine whether the powder was hazardous.

Arapahoe County Sheriff Grayson Robinson said it turned out the substance was not hazardous and not lethal, but that it will take days to determine what it was.

A second letter sent to a McCain campaign office in New Hampshire also raised alarm among staffers and prompted the evacuation of the office in Manchester. The Secret Service said the letter had a Denver return address, and workers had heard about the Colorado incident.

However, authorities said the letter did not contain white powder and was a false alarm.

Andy Lyon of the Parker South Metro Fire Rescue Authority in Colorado said the Denver letter bore the return address of the Arapahoe Detention Center and the name of an inmate.

Lyon said the first line of the letter used threatening language, but he declined to give details.

Seven people drove themselves to Colorado's Sky Ridge Medical Center, but none showed symptoms of exposure to a toxic substance, hospital spokeswoman Linda Watson said.

Twelve people were quarantined outside the Colorado office, including three police officers, two firefighters and seven civilians.

Bruce Williamson of the Arapahoe County Sheriff's Department said a hazardous materials team was inspecting the building.

Williamson said authorities took the incident "very seriously" because the Democratic National Convention begins Monday in Denver.

With files from the Associated Press