Threatening letters containing an unidentified white powder were sent to Republican presidential candidate John McCain campaign offices in Colorado and New Hampshire on Thursday, officials said.
No injuries were immediately reported.
At least 19 people were examined at hospitals or were quarantined outside the office in Centennial, Colorado, a Denver suburb. The office was evacuated, and authorities were trying to determine whether the powder was hazardous.
A McCain campaign aide said the Manchester, New Hampshire, office had been evacuated and the staffers underwent medical examinations.
No other details were immediately available from New Hampshire.
Jeff Sadosky, a spokesman with McCain's national campaign office in Arlington, Virginia, confirmed threatening letters were sent to the two offices.
A federal law enforcement official told NBC News no hazardous substances were found in the Colorado letter.
A statement from the Manchester Police Department in New Hampshire said there was no positive reaction in the field test for toxic substances at the scene.
Law enforcement officials also told NBC News the Colorado letter was sent by an inmate in the Arapaho County, Colorado jail, a person described as "a prolific letter writer."
The New Hampshire office called authorities when it saw a letter with Denver as the return address. The official did not know if it was the same return address as the Colorado letter.
Both letters were mailed from Denver, which currently is under a heightened state of security because of the Democratic National Convention that begins there Monday.
In Colorado, 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, said Andy Lyon of Parker South Metro Fire Rescue Authority.
"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," Lyon said.
Malcolm Wiley, a spokesman for the Secret Service in Washington, said the agency was notified Thursday afternoon that a white substance had been found in the campaign office just south of Denver. A staffer opened the envelope, but it was not known whether any of the powder got on her hands.
Agents from the postal inspector's office were on the scene in Centennial. Officials with the FBI and the Arapahoe County, Colorado, Sheriff's Department did not immediately return calls.
Meanwhile, Florida authorities were investigating a threatening letter sent to Gov. Charlie Crist that contained a suspicious but nontoxic white powder.
The letter was intercepted Wednesday at the mail center at the state capitol, Florida Department of Law Enforcement spokeswoman Heather Smith said.
Investigators believe the letter was unrelated to the one received at McCain's office in Colorado.
The letter sent to Crist was immediately sent to the Florida Department of Health for analysis, and preliminary results showed the substance wasn't harmful.
