BISMARCK, N.D. — A man who was serving life sentences for killing four people at a North Dakota business in what authorities say was one of the state's most gruesome crimes has killed himself in prison, authorities said.

The North Dakota Highway Patrol said a trooper was called to the state prison in Bismarck at around 5:45 p.m. on Sunday “regarding a resident who had caused self-harm.” The trooper was told that Chad Isaak, 48, had been taken by ambulance to a Bismarck hospital, where he was pronounced dead at 6:24 p.m.

Authorities didn't immediately give a cause of death.

Prison spokeswoman Kayli Richards said in a statement Monday that state confidentiality laws prevent her from disclosing whether Isaak had been considered a suicide risk or held under any special conditions at the prison.

She said Isaac shared a cell but that “his roommate was not present at the time of the incident.”

Richards said Isaak had “one minor pending disciplinary report” but had no others since being locked up. She didn't elaborate.

Isaak was serving four life sentences without the possibility of parole for the April 2019 killings of RJR Maintenance and Management co-owner Robert Fakler, 52; and employees Adam Fuehrer, 42; Bill Cobb, 50; and Lois Cobb, 45. The Cobbs were married. No motive was given at the trial.

The four were shot and stabbed inside the property management company’s building in Mandan.

Isaak, a chiropractor and Navy veteran, lived at a Washburn property that the company managed, but authorities never established a motive for the killings.

Isaak was appealing his convictions.

Jesse Walstad, one of Isaak’s trial attorneys, said Monday that he didn't know any details about his former client’s death.

“We concluded our representation after trial and sentencing,” he said. “We are very sympathetic to his family, of course. We have no comment beyond that.”

During the trial, defense attorneys argued that investigators didn’t seriously consider other possible suspects, including people who had either been evicted, sued or fired by RJR.

Prosecutors showed security camera footage from numerous businesses that authorities said tracked Isaak’s white pickup truck from Mandan to Washburn on the day of the killings, along with footage from a week earlier that they said indicated the killer had planned the attack in advance.

Forensic experts testified that fibers on the clothing of the slain workers matched fibers taken from Isaak’s clothing, and that DNA evidence found in Isaak’s truck was linked to Fakler and possibly Lois Cobb.

Prosecutors presented the case as a puzzle in which all of the pieces pointed to Isaak, including a knife found in his washing machine and gun parts found in his freezer.

At Isaak's sentencing in December, Jamie Binstock, a daughter of Robert Fakler, told the court that Isaak is a “heinous individual” who should spend the rest of his life in prison and experience a “lifetime of suffering” and nightmares.

“I do not want your life to end. Your family does not need to suffer the same way we did,” Binstock told Isaak in court.

In a statement to the Associated Press on Monday, Binstock said, “My thoughts and prayers are with his family.”