CENTENNIAL, Colo. -- A threatening letter containing an unidentified white powder that were sent to John McCain's campaign office in Colorado Thursday afternoon appeared to be a hoax carried out by an Arapahoe County inmate, county officials said.

A hazardous material team that was examining the white powder found in the Centennial office said that it was not hazardous and not lethal, said Arapahoe County Bureau Chief Bruce Williamson.

"Our guys did not find any powder. There were maybe a couple of grains of something inside an envelope and they had to kind of work to get a sample," said Andy Lyon of the Parker South Metro Fire Rescue Authority. "It was described to me as maybe they found a couple of granules of something. It tested positive for protein, what was described to me as a weak positive. Well, protein could be a protein shake."

The Colorado letter appears to have come from Marc Harold Ramsey, a 39-year-old inmate at the Arapahoe County Sheriff's detention facility who has a history of sending threatening letters, Williamson said.

Ramsey has been held at the jail since Sept. 17, 2007, on a $350,000 bond, on allegations of felony menacing, harassment and felony second-degree assault on a peace officer.

Ramsey will face additional felony charges as a result of the hoax, Williamson said. The exact charges will be determined by the United States Attorney's Office.

The Colorado letter arrived in the mail Thursday just after 3 p.m. and the campaign immediately notified local and federal law enforcement authorities.

"We are taking all necessary precautions," said Jeff Sadosky, a spokesman with McCain's national campaign office in Arlington, Va.

Five McCain staff members were taken to area hospitals as a precautionary measure and 12 staff members were quarantined outside the Centennial office after the scare, said Williams.

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.

All patients were released Thursday night.

"We don't have anybody who is sick, we don't have anybody who are showing signs of sickness," Lyon said.

Sadosky wouldn't say if the letter was addressed to McCain or the campaign in general. He didn't disclose the contents of the letter, only saying that it was a "very threatening letter."

The campaign had put all regional McCain offices on highest alert for suspicious activity, he said.

A letter bearing a Denver postmark that raised alarm in Ne Hampshire turned out not to be a threat, but campaign advice, officials said Friday. Click here to read the full story on the New Hampshire letter.

The Denver threat comes four days before the Democratic National Convention opens in Denver.

Crista Huff, who volunteers at the McCain office in Centennial, said she was "very concerned about the health of the people who work here."

"I'm depressed at the state of America that people would do this harmful act --- whether it was actual nasty powder or fake nasty powder -- either way, it is so incredibly hostile," she said.

The Secret Service, the Army National Guard's Weapons of Mass Destruction First responder team, the FBI, the Arapahoe County Sheriff's Office, the South Metro and Parker Fire departments and the U.S. Postal inspectors were all at the scene trying to assess the risk, just four days before the start of the Democratic National Convention.

"Well, there's certainly a lot of extra stress going on, I should say as a result of the DNC," Williamson said. "But, yeah we're not treating it any differently because it's Sen. McCain's office because it may or may not be politically based or anything. It's just, this is what we do with all of them."