“I am writing to ask for a pardon because I am innocent and want to go home,” Mr. Dassey wrote in a letter to the governor in April.

In response, the governor’s pardon advisory board said in an unsigned letter dated Dec. 17 that Mr. Dassey’s application would not be considered because he was ineligible for a pardon and that the governor would not consider any commutations.

A pardon and a commutation are two different types of clemency, Ms. Nirider said. A pardon restores some of the legal rights that are taken away when someone is convicted of a serious crime; a commutation is a shortening of a sentence, she said.

According to the board’s letter, Mr. Dassey was ineligible to be considered for a pardon because it had not been at least five years since he completed his sentence; and he had not registered as a sex offender, as he was required to do. Mr. Dassey is serving a life sentence without the possibility of early release until 2048.

“Had the Board reviewed Brendan’s petition on the merits, it would have seen what more than 250 national experts and millions of ordinary people around the globe see: a terrible miscarriage of justice,” Ms. Nirider and Steven Drizin, Mr. Dassey’s lawyers and directors of the Center on Wrongful Convictions at Northwestern University’s law school, said in a statement on Friday.