CENTENNIAL, Colo. (NBC) - Two separate suspicious letters targeted the campaign headquarters of Senator John McCain. One letter, containing an unknown white powder, arrived at McCain's office in Centennial, Colorado while the other was sent to his office in New Hampshire.

The incident here sent a flurry of law enforcement and emergency personnel to McCain's Colorado office.

Firefighters responded to McCain's office shortly after three o'clock Thursday afternoon after campaign workers encountered a threatening letter that held an unknown substance.

Andy Lyon of the Parker-South Metro Fire Department says "we have not found anything hazardous in the initial testing and the initial examination that we did."

A federal law enforcement source tells NBC News the letter was mailed by an inmate in the Arapaho County Jail, someone they describe as a prolific letter writer. Federal investigators say whatever was in the letter wasn't dangerous.

Andy Lyon says "the letter certainly had some threatening language, so they had reason to fear, but we did not find, the initial report was white power, they did not find large quantities."

A McCain staffer drove herself and three other workers to Sky Ridge Medical Center in Lone Tree where hospital staff decontaminated and then placed them in an isolated area.

Linda Watson of Sky Ridge Medical Center says "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."

So far, the four workers exposed to the substance aren't showing any ill effects. State lab tests will determine what the powder is.

The inmate suspected of sending the letter has been identified as 39-year-old Marc Harold Ramsey. Ramsey has been in jail since September of 2007 on investigation of felony menacing, harassment, and second degree assault against a police officer.

He may now face federal felony charges for Thursday's incident.