A second letter sent to a McCain campaign office in New Hampshire initially was reported to contain a white substance. Authorities said that was a false alarm and there was no powder in that envelope.
At least 19 people were examined at hospitals or were quarantined outside the Colorado office while authorities tried to determine whether the powder was hazardous.
Andy Lyon of Parker South Metro Fire Rescue Authority said the return address on the envelope listed the Arapahoe Detention Center and the name of an inmate. He didn't release the name.
Lyon said the first line of the letter used threatening language, but he refused to give any details.
McCain's campaign had said the letter sent to the Manchester, N.H., office also contained a threatening language and white powder.
But Malcolm Wiley, a Secret Service spokesman in Colorado, said there was no powder in the New Hampshire envelope. He said he did not know about the content of the letter.
Wiley said the letter had a Denver return address, which alarmed staffers in Manchester because they had heard about the Colorado incident.
Jim Barnett, McCain's New England campaign manager, said it's unusual for the New Hampshire office to get a letter from Denver.
"That was really the only suspicious thing about the letter, and our national headquarters advised, out of an abundance of caution for our staff and volunteers, that we have the authorities check it out," he said. "We did and it was deemed safe."
A government official familiar with the investigation said the New Hampshire letter was a false alarm. The official said authorities believe the Denver letter was a hoax because it appeared to have been sent from a jail.
Both the New Hampshire and Colorado offices were evacuated.
Lyon said about 40 people were evacuated from the Colorado building, but it contains several offices and businesses, and he did not know how many people had been in the McCain office.
Seven people drove themselves to Sky Ridge Medical Center, but none showed any 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, Lyon said.
He said Park South Metro firefighters found very little powder when they arrived.
"There were maybe a couple of grains of something inside an envelope and they had to kind of work to get a sample," he said.
Bruce Williamson of the Arapahoe County Sheriff's Department said a hazmat team was searching the building. He said authorities believe the substance was confined within the structure.
Williamson said authorities took the incident "very seriously" because the Democratic National Convention begins Monday in Denver and McCain is the presumed GOP candidate.
Postal Inspector Jo Jan Henderson said agents from her office were at the scene. FBI officials did not immediately return calls.