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Authorities believe the man who fatally shot a Massachusetts doctor targeted him because he was upset over the death of his mother, whom the physician had treated, a senior Boston law enforcement official told NBC News on Wednesday.

Stephen Pasceri walked into Brigham and Women’s Hospital on Tuesday, requesting to see Dr. Michael Davidson, police said. Their discussion quickly escalated from an argument into loud yelling, presumably by Pasceri, said the official, who was briefed on the investigation. Hospital staff said they heard screaming and then gunfire.

Davidson, the hospital's director of endovascular cardiac surgery, was shot twice and died Tuesday evening. He is survived by his wife and three young children.

The leading theory among investigators is that Pasceri was upset about his mother’s death and deliberately targeted Dr. Davidson, who had treated her, the law enforcement source said. It was unclear when Pasceri’s mom died or what was her cause of death.

Dr. Davidson’s father-in-law told the Boston Herald that the doctor warned his colleagues to get out just before the shooting. State licensing documents show Davidson hadn’t been subject to any medical discipline, he didn’t have a criminal record and he had never had to make payments on any malpractice claims.

The hospital said it was "heartbroken to have lost" Davidson "to a senseless act of violence."

"Kind, compassionate and beloved by his colleagues and his patients, Dr. Davidson was a bright light and an outstanding cardiac surgeon who devoted his career to saving lives and improving the quality of life of every patient he cared for," the hospital said in a statement.

Pasceri doesn’t have a criminal history except for a minor alcohol infraction in the late 1970s, police said. He was found dead at the hospital from a self-inflicted gunshot wound.

Police say they had the crime scene secured 16 minutes after the first calls for help were made. Hospital staff and police had completed an active shooter training drill in the past 6 months, officials said.