PORTLAND — Lt. Richard McMunn went into the burning nuclear submarine, descending through intense heat and black smoke that obscured his vision.

When he reached the deck, he fell into a hole that was five or six feet deep. He felt his ribs breaking. The pain was so intense that he did not realize that he had also hurt his knee.

McMunn tried to crawl forward but was too injured to continue.

But the knowledge that he had less than 20 minutes of air left and the fear of dying in the blaze drove him to climb back out of the submarine.

“I come to you today as a fortunate man. Unlike victims of similar crimes, I am still alive,” McMunn said Wednesday during a detention hearing for Casey James Fury, who is charged with arson in connection with two fires at the Portsmouth Naval Shipyard in Kittery.

McMunn asked that U.S. Magistrate Judge John Rich III order Fury to be held without bail, saying that the defendant does not seem to understand how dangerous his actions were.

McMunn said he was confined to a chair for three weeks, unable to move or lie down, was not able to walk for a month and is still bruised. He said the left side of his rib cage is permanently disfigured and that his knee will likely require surgery.

Rich determined that Fury, 24, of Portsmouth, N.H., should not be released because there were no conditions that could adequately ensure the safety of others if he were free.

Rich noted that Fury witnessed the consequences of the first fire, set inside the USS Miami on May 23, and yet apparently set another fire on June 16 outside the sub.

The judge also expressed concerns about Fury’s ability to cope with anxiety, given that he will be under scrutiny as the case moves forward and faces a possible maximum penalty of life in prison.

The May 23 fire caused $400 million in damage to the submarine.

Seven people were injured in the fire, which firefighters battled for 12 hours. The Senate Defense Appropriations Subcommittee on Wednesday approved a defense spending bill that includes $150 million for repairs to the USS Miami.

The smaller fire on June 16 was set to alcohol wipes in the dry dock cradle that holds the submarine.

Fury, a civilian who worked at the shipyard as a painter and sandblaster, told investigators that he set the June 16 fire because he was anxious and wanted to leave.

He had become anxious after texting a former girlfriend because he believed she was seeing another man, according to an affidavit filed in the case.

While Fury initially denied involvement in the May 23 fire, he eventually said he had set it to get out of work, according to the document.

During the hearing, Special Agent Brett Boley of the Naval Criminal Investigative Service testified that Fury had also confessed to a false fire alarm pull on June 19.

“He said he didn’t think his boss would let him get out of work if he said he wanted to go home,” Boley said.

Federal Defender David Beneman compared Fury’s behavior to a middle school student who pulls a fire alarm to escape a situation without understanding the possible danger.

Beneman noted that Fury sought in-patient mental health treatment — he checked himself in for two days following the June 16 fire — and is doing better since changing his medications.

Gregory Giblin, a federal probation officer, had recommended that Fury be released on bail. James and Margaret Fury, the defendant’s father and stepmother, testified that Fury could stay with them at their Eliot home and that they would look after him.

But Assistant U.S. Attorney Darcie McElwee said Fury put his desire leave work early above the lives of the people at the shipyard and Navy property. She said he was well aware of the damage caused by the first fire.

“What kind of person knows all that and sets another fire so he can leave early? I think that speaks volumes,” she said.

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