A man from upstate New York pleaded guilty to charges that he mailed 21 death threats with white powder in the envelopes over 15 years to U.S. Senator John McCain, a local high school and others, a U.S. attorney's office said on Friday.

Federal prosecutors argued that Brian Daniel Norton of Cicero said he was guilty in U.S. District Court in Utica Friday to two felony counts of conveying false information and hoaxes. He could face up to 10 years in prison and a $250,000 fine at his sentencing on April 10, 2015 by U.S. District Court Judge David Hurd. He will be detained until his next court appearance.

Between 1997 and 2012, Norton mailed letters with white powder that he claimed to be anthrax or ricin, as said in a statement from the U.S. Attorney for the Northern District of New York. Ricin and anthrax are extremely poisonous, but the powder found in the letters was analyzed and found to be harmless, typically baby powder or detergent.

Court papers also said Norton wrote phrases on the mail, including one alumni donation card: "Praise Allah for Sept. 11. You are next."

"The paper and other materials you are holding in your hands have been thoroughly contaminated with anthrax spores," another letter read. "Hopefully you and those around you will soon die a slooow painful death from anthrax."

The now 59-year-old also targeted former Secretary of Defense Robert Gates and former U.S. Rep. Ann Marie Buerkle of New York. Norton also sent threatening letters to Bishop Ludden High School and Le Moyne College, both of which are in Syracuse.

The FBI had Norton under surveillance since discovering his fingerprint on a letter in 2010 to Bishop Ludden. A second print was found on an anthrax greeting card, connecting Norton to the string of threatening mail, according to Daniel Capone of the FBI. His fingerprints were in the police systems from a 1976 arrest for harassment.