The man, Daniel L. Beckwitt, did not testify during his trial, in which he was convicted in April of second-degree murder and involuntary manslaughter. But he made emotional statements at his sentencing and apologized to the parents of Askia Khafra, the worker who died while digging the tunnels on Sept. 10, 2017.

Khafra, 21, had been carving out hundreds of feet of passageways under Beckwitt’s home in Bethesda, Maryland, northwest of Washington, when a fire broke out.

When firefighters arrived, they discovered Khafra’s “naked, charred body” in the basement of Beckwitt’s home, and a hole in the floor that dropped about 20 feet down and fanned out into tunnels spanning about 200 feet, according to court documents.

The state medical examiner determined that the cause of death was smoke inhalation and thermal injuries.

Judge Margaret Schweitzer of Montgomery County Circuit Court sentenced Beckwitt to 21 years in prison, but she suspended all but nine. Sentencing guidelines called for 10-21 years in prison, though second-degree murder in Maryland carries a maximum possible penalty of 30 years.

Schweitzer told Beckwitt, 28, that his “intellectual arrogance” had led to the tragedy, The Associated Press reported. “You thought that everything would be fine because you were very smart,” she said. “You thought you could fix everything.”

Ramon Korionoff, a spokesman for the Montgomery County State’s Attorney’s Office, said in a statement: “Today, defendant Daniel Beckwitt was held accountable in the death of Askia Khafra. While this sentence will not bring back Askia, it does ensure Beckwitt has years to think about the depraved-heart murder of this beloved young man.”

Robert Bonsib, a lawyer for Beckwitt, has maintained that the death was a tragic accident.

“The judge’s sentence in this matter was fair considering the jury verdict and was below the sentencing guidelines,” he said in an emailed statement Monday evening. “However, we continue to maintain that the guilty verdicts are not supported by the evidence in this matter, as this was a purely accidental fire — not a criminal act.”

He added that Beckwitt had already appealed the decision.

During the trial, Bonsib said that Beckwitt, who had made profits “in the seven figures” over the years by trading stocks, had been concerned about nuclear weapons and tensions with North Korea when he decided to build a secure location for himself.

Court documents said that Beckwitt made great efforts to keep his underground bunker secret. On some occasions he gave Khafra a ride from Silver Spring, Maryland, to the house in Bethesda but had him wear blackout glasses so he would not know exactly where he was working.

Khafra posted a photograph of himself, underground and wearing safety gear, on social media while he was doing the work.

Prosecutors said that Beckwitt’s home was dangerous because it was difficult to escape in case of emergency. Court documents said the house was “maintained in ‘hoarder’ conditions (immense piles of garbage and discarded items strewn throughout the home) with narrow mazelike pathways throughout.”

Prosecutors said that while excavating in September 2017, Khafra detected a smoky smell and sent a text message about it to Beckwitt, who eventually tried flipping a circuit breaker. Bonsib said that after the fire broke out, Beckwitt tried to rescue Khafra but could not because he was overcome by the smoke.

During the sentencing Monday, Khafra’s mother, Claudia Khafra, said that the death of her son “has left me broken,” The Associated Press reported. “I am constantly plagued by feelings of emptiness.”

This article originally appeared in The New York Times.