A Massachusetts inmate pleaded guilty to attempting to hire a fellow prisoner to murder three people, officials say.

Mason Stickney, 21, pleaded guilty on Wednesday to three counts of using a facility of interstate commerce in the commission of murder for hire, according to the U.S. Attorney’s Office for the District of Massachusetts.

Prosecutors say Stickney approached another inmate while at Essex County House of Corrections in October 2017, looking for help killing a police officer, a restaurateur from New Hampshire and a student. He also tried to have an Essex County prosecutor kidnapped and her relatives tortured. Their names were not listed in legal filings.

The inmate whose help he solicited -- he was convicted of child rape in 1992 and was awaiting trial on assault charges -- told the authorities and was instructed to give Stickney the phone number of a purported hitman, really an undercover FBI agent, according to a court filing viewed by The Boston Globe.

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The newspaper said Stickney told the undercover agent to shoot the restaurateur with a gun fitted with a silencer. He was to then cut off the hands before disposing of the body at sea, so he could leave fingerprints on a separate gun used to “ambush and murder” the officer.

Stickney also wanted the agent to “shoot and decapitate” the college student and discard his remains in the water, the filing says.

The inmate told authorities that Stickney planned to have a state prosecutor “kidnapped and put in a trunk, and the hell scared out her,” the court filing states. She was assigned to prosecute Stickney’s case and he “wanted a year or under on his sentence.”

During conversations with the agent, Stickney threatened the prosecutor’s family, demanding that he “grab the DA’s niece and nephew, torture them if you don’t mind, strip them nude basically, and tie them up and record them ‘Aunt (first name of ADA), help me!’, like little (expletive), and the, just you know, kill them after but don’t tell her you killed them,” the filing said.

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According to the filing, the agent was supposed to show the video to the prosecutor and “tell her I want no more than a year and if she snitches or if she gives me more than a year, the kids die.”

Stickney promised the would-be hitman $10,000 after he got out of jail, and to “get rid of three people for you guys.” The potential targets were never harmed.

Each charge carries a sentence of up to 10 years in prison and a fine of $250,000. He faced a number of charges at the time he was making his plan, including assault and battery with a dangerous weapon, stalking, destruction of property.

He is scheduled for sentencing in June.

Fox News’ Michael Sinkewicz contributed to this report.