"I wanted to try to relate to him in the letters," he said.
The letters stopped abruptly, and Spiro said he believes someone told Chapman not to write to the police department anymore. An injury ended Spiro's job as a New York police officer in 1983, but he said he kept the letters in a file for more than 30 years. He decided to sell them in part to pay off hefty medical bills from cancer and other illnesses and because he thought they should be in the public domain.
"I wanted to publicize them to the world because they're part of history," he said.
Spiro, 66, had no contact with Chapman after the last letter.
Chapman, 57, was denied parole in August for the seventh time. During the hearing, he again expressed remorse for Lennon's killing and spoke of his Christian faith.
Lennon's widow, singer and artist Yoko Ono, still lives in the Dakota apartment she shared with Lennon, who was 40 when he died. She was celebrating her 80th birthday with a concert in Berlin and couldn't be reached for comment Monday.
Associated Press writer Andrew Dalton contributed to this report from Los Angeles.