Two of Sen. John McCain's presidential campaign offices were evacuated Thursday after receiving suspicious letters, one of which contained an unidentified white powder.
A government official said a letter sent to the office of the presumptive Republican presidential nominee near Denver contained powder, but was being treated as a hoax and was thought to have been sent by an inmate at a nearby jail in Arapahoe County.
A letter sent to the office in New Hampshire did not contain powder, but aroused suspicions of staffers who were jittery after the Denver-area office received its letter.
Secret Service spokesman Eric Zahren said the two letters are unrelated.
Despite the likelihood of a hoax, staffers at all the Arizona Republican's campaign offices nationwide were told not to open letters.
"Let's be cautious and not open mail tomorrow unless we clearly know the source," according to a campaign e-mail that was sent to staff members and obtained by The Washington Times. "Let's see what comes of this."
Staffers at Mr. McCain's offices in Centennial, Colo., received a letter Thursday afternoon that contained a white powder and a warning not to open it, according to Mr. Zahren. The letter did not threaten any particular individual.
The staff was quarantined, the building evacuated, and a source said four people checked themselves in to hospitals.
"We have put all of our offices on highest alert," McCain spokesman Jeff Sadofky told Fox News.
The Secret Service, along with the FBI and the Arapahoe County Sheriff's Office, were investigating.
Mr. Zahren said the other letter was received at Mr. McCain's campaign office in Manchester, N.H. That letter did not contain powder, but further information about it was not available late Thursday.
Denver is under a heightened security alert because of next week's Democratic National Convention.
The letters come shortly after the suicide of Bruce Ivins, an Army microbiologist who the FBI said was responsible for the 2001 anthrax mailings that killed five people, sickened 17 and further frayed the nerves of a nation reeling from the Sept. 11 terrorist attacks.
It was not clear whether the letters sent to Mr. McCain's campaign offices included any references to anthrax.