A Somerville man imprisoned for a quarter of a century could be freed soon after a Middlesex Superior Court judge ruled DNA testing supports his claim he did not participate in a 1986 murder, a brutal attack that left the victim to die slowly in the parking lot of an abandoned supermarket.

“He is ecstatic,’’ said Boston lawyer Dana Curhan, who has served as ­Michael J. Sullivan’s appellate attorney since 1991 through the Committee for Public Counsel Services Innocence Program.