BOSTON -- The murder at Massachusetts General Hospital of a doctor by a worker who then committed suicide is solved, but the motive will never be known.

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District Attorney Describe's Investigation's Findings

Saying he is prohibited from discussing the medical health records of the killer, the Suffolk County district attorney released the investigation's findings that rule out a number of theories for the murder/suicide on Thursday, Newscenter 5's David Boeri reported, but couldn't reveal a reason why.

"We do not conclusively know the motive for the shootings. And we may never know. Those of us who investigate homicide cases and those of you who cover them in the media know that you often cannot attribute a sensible motive to such a senseless act of violence," said Daniel Conley, Suffolk County district attorney.

Inside his office at the hospital that day in April she shot Dr. Brian McGovern four times in his chest, his head, his neck and his hand. Then, secretary and receptionist Colleen Mitchell shot herself in the head with a gun she bought in a Virginia pawnshop.

She only worked at MGH for a few months, she was not his personal secretary and investigators did not find anything personal connecting murderer to victim.

"There is absolutely nothing that suggests that there was a personal relationship between the two," Conley said.

Mitchell was 51 years old and recently came from Virginia. She lived on Beacon Hill, where police found rounds of ammunition and a couple of bottles of prescription antidepressants in her apartment.

"Some media outlets reported an un-sourced rumor that Dr. McGovern may have caught Mitchell stealing drugs from the hospital and confronted her. There is absolutely nothing to support that theory," said Conley.

At MGH, investigators found no disciplinary cases and no professional disputes involving Mitchell. They found little more than the .38 caliber handgun she had bought in Virginia three years ago and brought to the hospital without a license to carry the day she killed herself and the doctor.