The Colorado man who sent a threatening letter and white powder to Republican White House hopeful John McCain's Colorado campaign headquarters put his name and address on the envelope, court documents showed on Friday.

The letter briefly sparked alarm among staffers who were hospitalised and placed under quarantine late Thursday, but authorities quickly determined that the powder was not dangerous.

Marc Harold Ramsey (39), signed the letter to McCain "Akeem Ramsey El" but wrote his correct name, ID number and address at the Arapahoe County Detention Facilities on the envelope, an affidavit released by the US Department of Justice said.

Ramsey was charged with mailing a threatening communication and was expected to be transferred to federal jail to make a court appearance, the Justice Department said.

A Secret Service agent had earlier identified Ramsey as the letter writer and said he was behind bars on charges of harassment and assault of a police officer.

"He is on record with us and he is known as a prolific letter writer," Secret Service spokesman Ed Donovan told AFP.

Ramsey admitted sending the letter to McCain "to express his feelings about the United States government," according to the affidavit.

"Ramsey stated that his father was in Vietnam during the same time as Senator McCain and that the government takes care of Senator McCain but not his father, who suffers from Agent Orange," the document said.

"Senator McCain, If you are reading this then you are already dead! Unless of course you can't or don't breathe," Ramsey's letter said, according to the affidavit.

"There are numerous substances which are deadly for humans to inhale. There are just as many time periods for signs of illnesses to show, by which time it's to (sic) late.

"Who expects to develop cancer 40 years after Vietnam? Only those that knew the risk and side effects of Agent Orange ... You're out of time. Allahu Akbar. Akeem Ramsey El," it read.

The powder fell on the leg of the office manager who opened the letter, the affidavit said. She and four others checked into area hospitals to be decontaminated, and nineteen people were decontaminated on site, it said.

Following the 11 September 2001 attacks, threatening letters containing anthrax were sent to prominent politicians and journalists, and five people died after coming in contact with the substance.

AFP