"He did not seem to understand the concept of seniority," he said.
The most senior drivers pick the best routes, leaving the remainder for less senior workers, Montminy said. As a result, Thornton believed he was subjected to racism, he said.
The union said last year that 14 of 69 dock workers, or 20 percent, were racial minorities - four black, nine Hispanic, one Asian.
Thornton shot eight people within three minutes Aug. 3 before killing himself. Immediately before the shootings, he was fired for stealing beer.
Montminy said investigators found no evidence of racism, but were told that a drawing of President Barack Obama with a noose around his neck was seen on a bathroom wall months before the shootings and was quickly removed by the company.
Police say the shootings appeared random. Thornton, 34, shot several workers with whom he had disputes and those with whom he did not have much contact, Montminy said.
Thornton did not shoot two disabled workers, one who was in a wheelchair and the other who is mentally handicapped, "when clearly he had the opportunity to shoot them," the police chief The report describes how Thornton moved quickly through the office and warehouse, shooting co-workers at close range. The first 911 call police received was from Steve Hollander, vice president of Hartford Distributors.
Montminy said Thornton held a gun in one hand and his lunch box, which he used to smuggle in the gun and ammunition, in the other hand.
"People started to flee the building from every direction," Montminy said.
As soon as police arrived, Thornton barricaded himself in an office and fatally shot himself.
Montminy said the greatest challenge in the investigation, which he said ended just a few weeks ago, was to examine the contents of Thornton's computer and cellphone. Police were searching for evidence to back his claims of racism, the police chief said.
Officials did not include crime scene photos showing the victims, but parts of the report were gruesome. For example, one victim was shot in the head and his body was badly burned by a fire started by a forklift that was still running after its driver was shot.