"On Friday, we will go to court with Jimmy Parker, who will waive indictment and plead guilty to one count of accomplice to the second degree murder of Susanne Zantop," according to a statement from Parker's lawyers Cathy Green and Philip Utter. First-degree murder charges against Parker are expected to be dropped.
The accomplice charge carries a sentence of up to life in prison. The agreement still must be approved by a judge, and a hearing was scheduled for Friday.
In a case that drew international attention, James Parker, 17, and his friend Robert Tulloch, 18, both of Chelsea, Vt., were charged with the Jan. 27 stabbing of Half Zantop, 63, and his 55-year-old wife, Susanne. Investigators found the couple, who were stabbed multiple times in the face, neck and chest, in pools of blood at their secluded Hanover, N.H., home. Tulloch remains charged in both deaths.
The teens disappeared from their hometown in February, sparking a manhunt that ended with their capture at an Indiana truck stop.
The Boston Globe and the Boston Herald said Monday a family friend told them the agreement included a sentence of 25 years to life in prison, and Parker would testify against Tulloch.
Tulloch's lawyer, Richard Guerriero, took Monday's development in stride.
"We welcome any admissible testimony that helps explain the tragedy," Guerriero said. "The right way to resolve a case is by testimony in court rather than speculation on the front page."
Parker, who was 16 at the time, had been charged as a juvenile. But prosecutors worked to have him certified as an adult after he turned 17 last month.
Meanwhile, Tulloch's lawyers Friday filed court papers saying they would "rely on the defense of insanity at trial."
The motion was filed after the deadline set by New Hampshire law but Guerriero, claimed it took a psychiatrist longer than expected to evaluate Tulloch due to "Robert's intelligence and his particular personality."
Grafton County Superior Court Judge Peter Smith is expected to consider later this week whether Tulloch's insanity defense will be allowed in his trial, set to begin April 8.
The teens were arrested following an investigative trail that led from the Zantops' home to Chelsea, roughly 20 miles away. Investigators found two knife sheathes at the Zantop residence as well as a bloody footprint.
Court documents revealed police found two military-style knives in Tulloch's bedroom with blood matching Susanne Zantop's. Knife purchases over the Internet pointed police to Tulloch, according to court records.
New Hampshire Attorney General Philip McLaughlin has not given a motive for the killing.
©MMI CBS Worldwide Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press and Reuters Limited contributed to this report