SKOWHEGAN — A Waterville man who pleaded guilty earlier this year to a federal charge of conspiring to attack a Shia mosque in the Chicago area was indicted Thursday by a Somerset County grand jury on a count of aggravated assault.

Xavier J. Pelkey, 19, is facing the assault charge because he used a dangerous weapon, a chair, to injure another inmate in January at the Somerset County Jail, according to a copy of his indictment released Friday.

Pelkey was arrested by the FBI in February 2022 at his apartment in downtown Waterville after federal authorities said he was found with homemade explosive devices, according to a federal criminal complaint.

He pleaded guilty in April in federal court in Bangor to conspiring with terrorists to conduct a mass shooting at the mosque, federal prosecutors said at the time. His plea was part of an agreement with prosecutors that brought with it a 15-year prison sentence followed by a lifetime of supervised release. Pelkey entered his plea on a charge of conspiracy to provide material support to terrorists.

He conspired with two minors, one in Chicago and another in Canada, in November 2021 to unleash the shooting at the mosque, prosecutors have said in court records.

The three were adherents of a radical form of Sunni Islam and supporters of the terrorist organization Islamic State, also known as ISIS, authorities previously said. The three formulated plans over Instagram.

It wasn’t clear Friday why Pelkey as a federal prisoner was being held at the Somerset County Jail in January, when county officials say he injured the other inmate. But Mike Mitchell, chief deputy of the Somerset County Sheriff’s Office, said the county is one of the few ones in the state with a federal contract allowing them to hold federal prisoners. Somerset at any one time has about 40 federal offenders under detention, he said.

Pelkey may have been held at the Somerset jail up until his plea in federal court in April, after which he likely would have been be transferred to a federal facility. Online jail records indicate Pelkey was not being held at the Somerset County Jail on Friday.