By Joseph Ax

(Reuters) - A 20-year-old North Carolina man accused of murdering his neighbor and offering to pay someone to kill his own parents pleaded guilty on Tuesday to plotting mass shooting attacks in the name of Islamic State.

Justin Sullivan admitted to one count of attempting to commit an act of terrorism transcending national boundaries in federal court in Asheville, North Carolina.

The plea agreement calls for a sentence of life in prison.

U.S. prosecutors said Sullivan conspired with Junaid Hussain, a British hacker who was an active online recruiter of people to carry out attacks on behalf of the militant group Islamic State. Hussain was killed by a U.S. military air strike in Syria last year.

Islamic State has carried out or inspired attacks in the United States, Europe and elsewhere, including the June massacre of 49 people in an Orlando nightclub.

Sullivan had discussed his plot online with an undercover federal agent, making plans to buy a semi-automatic rifle at a gun show and discussing potential targets. Sullivan said he would use the rifle at a concert, bar or club in an effort to kill as many as 1,000 people.

At Sullivan's request, the undercover agent sent a homemade silencer to his home, which was opened by Sullivan's mother, prosecutors said. When his parents questioned him about the silencer, Sullivan offered to pay the agent to kill them lest they interfere with his plans, according to prosecutors.

Sullivan also told investigators he had stolen his father's rifle and hidden it, authorities said. The rifle was used to murder John Bailey Clark, Sullivan's neighbor, and North Carolina state prosecutors indicted him for murder in February.

Sullivan did not admit to the murder as part of his plea, but U.S. prosecutors said they may present evidence of the crime at his sentencing.

State prosecutors have previously said they plan to seek the death penalty for Sullivan on the murder charge.

(Reporting by Joseph Ax; Editing by Tom Brown)