Updated

A North Carolina man pleaded guilty on Monday to the 2014 shooting death of his elderly neighbor, just weeks after receiving a life sentence for plotting a massacre on behalf of the Islamic State.

Multiple media outlets report 21-year-old Justin Nojan Sullivan admitted Monday in a Burke County courtroom that he shot and killed 74-year-old John Bailey Clark in December 2014 as he slept in his bedroom.

Sullivan was tied to his neighbor's slaying after federal authorities launched a terror investigation, which was prompted by a 911 call in April 2015. His father told police that the young man was destroying religious items in their home in Morganton.

Weeks of communications with an undercover informant followed. He was arrested at the family's home on June 19, 2015, after he allegedly communicated with an Islamic State figure that same month via social media about making a video of a terrorist attack. Sullivan was accused of planning to use an assault rifle and silencer to carry out an attack at a concert, bar or club, which he believed would kill as many as 1,000 people.

Investigators said Sullivan told them he planned to carry out the attack when his parents would be out of town. Sullivan also allegedly offered an undercover FBI employee money to kill his parents to prevent them from interfering.

Forensic tests on his father's .22-caliber rifle later determined that the same gun had been used to shoot Clark in the head. Authorities accused Sullivan of taking the man's money to finance the act of terror.

A Burke County grand jury indicted Sullivan on a murder charge in the death of Clark, who lived a few doors down from him, said District Attorney David Learner.

Sullivan's lawyer, Victoria Jayne, said Burke County prosecutors agreed not to seek the death penalty. She couldn't be reached for additional comment on Monday, but she told The Charlotte Observer that he pleaded guilty because "Justin just wants to get this over with," before he is assigned a federal prison to serve his ISIS sentence.

"He's sort of anxious at this point of getting into the federal Bureau of Prisons. He's very nervous about that," Jayne told the newspaper. "He is socially awkward and he is socially immature."