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Authorities have released additional details of the active shooter incident at a Molson Coors complex in Milwaukee that left five employees dead before the killer reportedly turned a gun on himself.

The attacker—identified as a 51-year-old man who worked as an electrician at the company—entered the complex and opened fire with two handguns, police sources said.

A co-worker who asked not to be identified for fear of being disciplined said the suspected gunman believed he was being discriminated against because he was African American and that he frequently argued with at least one of the victims, a fellow electrician, the Milwaukee Journal-Sentinel reports.

The co-worker said the suspect often watched movies on his phone during the day, which the other electrician took issue with, and that the two accused each other of going into each others' offices and stealing tools or tampering with computer equipment.

About a year ago, the employee said, the suspect started saying he believed brewery workers were coming into his home, bugging his computer and moving chairs around. "Things just started getting weird. But he was dead serious about it," the co-worker said.

Multiple police agencies responded to 911 calls of a shooting at the facility at 1400 hours local time. Arriving officers found employees performing CPR on a single gunshot victim, NBC News reports.

Police reportedly worked for hours to clear the more than 20 buildings in the complex where more than 1,000 people work. They eventually found the suspected gunman dead of an apparent self-inflicted gunshot wound.

Related: 5 People Killed in Active Shooter Attack on Milwaukee Brewery

"There were five individuals who went to work today, just like everybody goes to work, and they thought they were going to go to work, finish their day and return to their families. They didn't—and tragically they never will," Milwaukee Mayor Tom Barrett said.

President Donald Trump said, "We send our condolences. We’ll be with them. And it is a terrible thing. Terrible thing. So our hearts go out to the people of Wisconsin and to the families."

None of the names of the five have been released. Remarkably, it is believed that no one was wounded other than the five individuals who were killed.