CONCORD, N.H. (AP) – Nearly a week after a judge seemed unmoved by his claim of self-defense, Eric Windhurst has reached a plea agreement in the shooting death of his girlfriend’s stepfather, a case that went unresolved for 20 years.

The self-employed contractor from Hopkinton was a 17-year-old high school student and skilled marksman when he allegedly targeted Danny Paquette from a distance, using a hunting rifle to kill him with a single shot through the heart in 1985.

Arrested and charged with first-degree murder last December, Windhurst, now 38, claimed he was protecting his 15-year-old girlfriend, who said Paquette had raped and abused her in the past; Windhurst feared Paquette would harm her again. But prosecutors disputed that, and a judge said he was strongly inclined to agree with them. Windhurst is scheduled to enter a plea and be sentenced on Monday.

For years, Paquette’s death was a cold case. It was featured on “Unsolved Mysteries,” though his family had suspected Windhurst since at least 1992, when they received anonymous letters and a phone call saying Windhurst was the killer.

“This has been traumatic – heartbreaking – that so many people were well aware of Eric Windhurst’s activities and were willing to keep their mouths shut,” said Victor Paquette, Danny’s brother.

Investigators got a break in 2004, when the stepdaughter, Melanie Paquette Cooper, agreed to cooperate. A taped phone call between Cooper and Windhurst led to his arrest. Court documents from that investigation also showed that Cooper said she was present when Windhurst allegedly shot Paquette.

Windhurst’s trial was scheduled to start on Aug. 28. But a notice filed Wednesday in Merrimack County Superior Court said Windhurst will appear for a plea and sentencing hearing Monday before Judge Robert Lynn in Nashua. The notice does not indicate what charge Windhurst will plead to, nor what sentence was negotiated.

Victor Paquette said the attorney general’s office informed him Wednesday of the plea, but he did not know its details.

“As I understand it in conversation … he has agreed to plead guilty in exchange for a lesser charge, like second-degree murder of something like that,” Paquette said.

Paquette said he was angered by the plea. As recently as last Friday, prosecutors assured him they were confident of their prospects at trial, after Lynn agreed to allow evidence suggesting that Windhurst shot Paquette because he was angry after hearing that his own father may have sexually abused girls.

Paquette accused the attorney general’s office of caving to pressure to protect the Windhurst family – which is prominent in Hopkinton.

Cooper’s aunt, Merrimack County Judge Kathleen McGuire. McGuire, who also is a former assistant attorney general, was seeking guardianship of Cooper when Paquette was killed. At the time, Cooper, whose mother had divorced Paquette, claiming abuse, had recently returned to New Hampshire from Alaska. She was living with her aunt and uncle.

Victor Paquette has said all along that Danny never physically or sexually abused any members of his family. He also doubted his brother knew of Cooper’s return.

“It does not surprise me that the state … is willing to throw my brother away in an effort to keep this out of court to protect a lot of wealthy, politically connected people, meaning the Windhurst family and Judge McGuire, as well as, you know, casting a shadow on the attorney general’s office,” Victor Paquette said. “Right now, I am thoroughly disgusted with the attorney general’s office that they would even entertain a motion to plea for a lesser charge.”

The attorney general’s office declined to discuss the details of the plea agreement. But senior assistant attorney general Jeff Strelzin disputed Paquette’s other claims.

“We brought this 20-year-old case forward to seek justice,” he said. “Every decision in this case has been based on our objective review of the facts and the law, and no other factors have influenced our decisions.”

No one answered the phone Wednesday at Windhurst’s father’s residence in Hopkinton.

Windhurst’s lawyer, Mark Sisti, declined to speak in detail about the agreement before Monday’s hearing.

“I believe it’s a reasonable offer and it puts some closure on this whole situation,” he said.

Cooper, who now lives in Evanston, Wyo., had agreed to testify against Windhurst and plead guilty to a felony charge of hindering apprehension for misleading investigators in 1992. Her lawyer declined to discuss whether Windhurst’s plea would affect her case. A female who identified herself as Cooper’s daughter said her mother was unavailable to speak to a reporter.