Prosecutors said Marc Harold Ramsey, 39, sent a letter to McCain's office in the south Denver suburb of Centennial reading, "If you are reading this, then you are already dead!"
The letter was signed Akeem Ramsey El, but the return address on the envelope had Ramsey's name and "Arapahoe County Detention Facility."
Ramsey, 39, is in the Arapahoe County jail awaiting trial on a charges of menacing, harassment and assaulting a peace officer.
U.S. Attorney Troy Eid said Ramsey told investigators he wrote the letter to express his views about the U.S. government.
"A death threat by no means is a legitimate form of political expression," Eid said.
Staffers at McCain's office opened the letter Thursday afternoon. At least 19 people were examined at hospitals or were quarantined outside the office.
A Colorado National Guard Civil Support Team determined the powder was not lethal at about midnight and everyone was sent home.
About 55 National Guard teams across the country respond to this type of incident. Three will be in Denver for the Democratic National Convention next week.
The teams respond to similar incidents about once a week, Guard spokesman Rick Breitenfeldt said. Thursday's letter received media attention because it involved a presidential candidate.
The Secret Service - which provides protection to the candidates - said a separate scare triggered by a letter sent to a McCain campaign office in Manchester, N.H., was a false alarm.
