VIRGINIA BEACH, Va. (AP) – A months-long independent probe into the Virginia Beach mass shooting has concluded, but it offers no clear answers as to why a city engineer killed 12 people in his workplace, according to findings released Wednesday.

The investigation, conducted by Chicago-based security company Hillard Heintze, found no warning signs by the shooter that could have helped the city prevent the May 31 tragedy.

The firm also said that the longtime city employee did not work in a systemically toxic workplace, which some people in this coastal city of nearly 500,000 had proposed as a possible factor.

What the investigation did reveal was a man who experienced the kind of pressures and life challenges that many people do.

DeWayne Craddock, 40, had gone through a divorce and was having trouble at work. He wrote emails on his work computer that went unsent but claimed he was unjustly disciplined.

But independent investigators were at a loss to explain how such “stressors” could have translated into violence.