DNA found at that scene also matched Mr. Gaff’s, prosectors said.

Jackie O’Brien, who was assaulted in 1979 by Mr. Gaff, said she had mixed feelings about the guilty plea.

“I wanted him to go through a trial,” she said in a phone interview on Tuesday.

Ms. O’Brien, 76, was 29 when Mr. Gaff assaulted her in a tool shed, where, according to court documents, he beat her with an air gun, bashed her head into the cement floor, slashed her palm with a knife and threatened to kill her.

Ms. O’Brien managed to fight off and flee Mr. Gaff, who was arrested a short time later, carrying adhesive tape and a gym bag that contained leather gloves, a bandage, stocking cap, face mask and a dildo, according to court documents. He was convicted of first-degree burglary and assault in the second degree, granted probation and placed on work-release, according to court documents.

“His punishment for what he did to me wasn’t severe enough,” Ms. O’Brien said.

“As soon as he got out of jail, he raped and sodomized those two little girls,” she added. “After all these years, 46 years, it still makes me cry — I wish, I wish I could have killed him.”

In an interview with The New York Times in 1995, Mr. Gaff, who had just completed a 10-year sentence for raping the teenage sisters at knife point, said he was stunned to learn prosecutors were seeking his indefinite confinement under a “sexual predator” law.

His sentencing is scheduled for May 13.