NBC News says the person is a prolific letter writer who was already in the jail. The Arapahoe County Sheriff's Office says 39-year-old Marc Harold Ramsey has been in jail since Sept. 17, 2007.
Sources also tell NBC that there is no link between the Colorado letter and another threatening letter that appeared on Thursday at McCain's office in Manchester, New Hampshire. That letter had a return address from Denver, but investigators do not believe the two incidents are connected.
Investigators say a staffer opened the envelope just after 3:15 p.m. at the office at 6334 S. Racine Circle near Arapahoe Road and Peoria Street in Centennial.
It had a white powdery substance inside, but authorities have determined the material was not hazardous and not lethal.
A McCain staffer drove herself and three other workers to Sky Ridge Medical Center where they were decontaminated on Thursday afternoon after they said a white powder was found in an envelope. Another staffer was also taken to the hospital at one point, according to the Arapahoe County Sheriff's Office.
According to the Secret Service and local firefighters, the envelope also contained a letter with a threat against McCain.
"Something that was written down that caused alarm," said Arapahoe County Sheriff's Office Bureau Chief Bruce Williams.
An Arapahoe County Sheriff's Department Haz-Mat team responded to the building and the employees went through decontamination showers at the hospital.
Andy Lyon with of Parker South Metro Fire Rescue Authority spoke with 9NEWS after the discovery of the substance.
"We have not found anything hazardous in the initial testing, in the initial examination we did," said Lyon. "We have no reports of anybody being sick. We have 12 people quarantined here on scene who were exposed - three of those law enforcement, two firefighters and seven civilians. None of them are showing any symptoms. Nobody's reporting any sickness."
Lyon says the people who went to the hospital did not report they were sick.
The Arapahoe County Sheriff's Office says the first deputy that responded to the scene was also quarantined as a precaution until it was determined the suspicious substance did not present a threat.
Sky Ridge says after the people at the hospital were decontaminated, they were placed in an isolated area.
"In any situation like this, when it's a little out of the norm, unusual, something that could create a hazard we really want to jump on it right away and make sure we're protecting the folks coming into see us but the patients and health care workers that we currently have in the hospital," said Linda Watson with Sky Ridge Medical Center.
"Our Haz-Mat team did not, said that they did not see any powder. The letter certainly had some threatening language so they had reason to fear, but we did not find - the initial report was white powder - they did not find any large quantities. It was described to me as they found maybe a couple granules of something. It tested positive for protein," said Lyon.
Lyon says there were also no positive tests for radiation and no tests at all that anything's hazardous.
They plan to transport the evidence to a state lab for more testing.
"Any time you send a threatening letter, and there was threatening language in the letter, that's a crime so law enforcement wants to take this very seriously," said Lyon.
He says the letter was hand-written.
Postal Inspector Jo Jan Henderson said agents from her office were at the scene in Centennial.
The road around the business was closed off, but Williams says no major traffic was being blocked.
He also says there was no danger to the environment around the building.
"Whatever it is, it's contained within the building itself," said Williams.
The Arapahoe County Sheriff's Office says Ramsey has been held for nearly a year on a $350,000 bond regarding allegations of felony menacing, harassment and felony second-degree assault on a peace officer.
As a result of this investigation, he could face a variety of federal charges.
The threats came four days before the Democratic National Convention opens in Denver.
