Investigators have used DNA evidence to identify one of the seven remaining unknown victims of John Wayne Gacy, a 1970s serial killer who raped and murdered dozens of teenage boys and young men and was labeled “the killer clown” by the news media, the authorities said on Wednesday.

The Cook County Sheriff’s Department in Illinois said that the newly identified victim was James Byron Haakenson, a 16-year-old boy who ran away from his home in St. Paul in the summer of 1976. The police found his body and 26 others in the crawl space beneath Mr. Gacy’s home in December 1978.

Mr. Gacy was among the most prolific serial killers in American history. From 1972 to 1978, he lured or forced 33 teenage boys and young men — some of whom he employed at his construction business — to his home in suburban Chicago, where he murdered them. He sexually assaulted most of his victims as he tortured them to death and buried all but six of them beneath his house.

Neighbors in Norwood Park Township, Ill., knew Mr. Gacy for his appearances at children’s parties where he performed as Pogo the Clown, a character he later painted or sketched numerous times while in prison.