The letter allegedly was mailed by an inmate at Colorado's Arapahoe County Jail, The Denver Post reported.

With the Democratic National Convention set to begin Monday, Denver is already on heightened security alert.

"As a precautionary measure, we have closed the office and staff are being examined by medical personnel," a McCain campaign representative told CBS News.

Several campaign workers at the office in Centennial, Colo., were taken to area hospitals, but none showed indications of being ill. Officials said the substance was not dangerous -- although U.S. Secret Service Agent Malcolm Wiley said late Thursday the substance had not been identified.

Wiley said Marc Ramsey, an inmate in the jail, sent the letter.

"He is a prolific writer of threatening letters," Wiley said.

A similar letter was delivered to the McCain campaign office in Manchester, N.H., Thursday. Wiley said the New Hampshire letter had a Denver return address, but it was not immediately clear whether Ramsey sent it.

Law enforcement authorities declined to comment on the contents of the letter received in Colorado, the Post said.