SYRACUSE (Reuters) - A New York man pleaded guilty to mailing more than 20 death-threat letters he claimed were laced with anthrax or ricin to schools and US officials and lawmakers, including Senator John McCain, between 1997 and 2012, a U.S. attorney's office said on Friday.

Brian Daniel Norton, 59, of Cicero, pleaded guilty in US District Court in Utica to two felony counts of conveying false information and hoaxes, according to a statement from the US Attorney for the Northern District of New York. "In court, Norton admitted he mailed more than 20 death threat letters that contained a white powder he claimed to be either anthrax or ricin," the statement said.

Both ricin and Anthrax can be deadly, but powder found in the letters was analysed and found to be harmless, the statement said.

The letters were sent to Bishop Ludden High School and Le Moyne College, both in Syracuse, and to US Senator from Arizona John McCain, then-Secretary of Defense Robert Gates, and former Congresswoman Ann Marie Buerkle.

Court papers said Norton wrote phrases on the mailings. On one alumni donation card he said: "Praise Allah for Sept. 11. You are next," local media reported, referring to the attacks on the United States.

Norton faces a maximum of five years in prison and a US$250,000 (S$312,575) fine. He is being detained pending sentencing, scheduled on April 10 in Utica.