LANCASTER, N.H. — The owner of a New Hampshire gunpowder plant where two workers were killed in a 2010 explosion was motivated by profits, failed to ensure employee safety and should be held responsible for the deaths, a prosecutor said Monday as the man’s manslaughter trial opened.

The lawyer for plant owner Craig Sanborn, 64, of Maidstone, Vt., countered that the explosion could have been caused by employee error.

Defense attorney Mark Sisti emphasized that Sanborn was in North Carolina the day of the explosion and had no control over the plant or conditions that led to it.

Sanborn is charged with manslaughter and reckless homicide in the explosion at his Black Mag plant in Colebrook that killed Donald Kendall, 56, of Colebrook, and Jesse Kennett, 49, of Stratford. They had been hired a month earlier.

The force of the explosion shook nearby buildings and sent plumes of smoke into the air. Dozens of homes were evacuated and firefighters couldn’t get close to the blaze for several hours because ammunition was exploding.

Prosecutor John McCormick told jurors that Sanborn was reckless in manufacturing, testing and storing the black powder and failed to adequately train and protect workers. He told them Sanborn was also trying to meet conditions of an ambitious and lucrative contract for which he’d already received a $300,000 down payment.

“The case before you is brought to you by an age-old motivating factor and force — that being greed,” McCormick said.

The defense told jurors that even the experts hired by prosecutors couldn’t pinpoint the cause of the explosion.

Sisti said the explosion could have been sparked by a stray piece of metal that caused friction inside a machine where the gunpowder was being processed, a worker smoking in violation of the rules or a machine that was running too fast.

“You have a man on trial who didn’t have control because he was 1,000 miles away,” Sisti said. “It could have been an out-and-out accident.”

Mark Porter, who was the first witness to testify, said he resigned from the plant after six days over safety concerns. He said the only instructions he received were to run if there was a fire and that he shared with Sanborn his concern that the plant was “very dangerous.” Porter quit nine days before the May 14, 2010 explosion.

Porter said on cross-examination that Sanborn yelled at him and he feared the confrontation would escalate, so he signed a resignation letter saying he was quitting because the gunpowder irritated his eyes.

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