An employee opened fire on Wednesday in a brewery complex in the US state of Wisconsin, killing five fellow workers before taking his own life.

The gunman from the city of Milwaukee was identified as a 51-year-old man who died of a self-inflicted gunshot wound, police said.

"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.

"It is a horrible, horrible day for the employees here," Barrett told reporters. "A very rough day for anyone who is close to this situation."

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Police tweeted there was no longer an active threat.

Authorities offered no immediate motive for the attack and did not release details about the shooter or how the shooting unfolded. None of the victims was identified. Police, who were still contacting relatives, said identities would not be released for at least 24 hours.

No one was wounded beyond those who were killed, authorities said.

President Donald Trump addressed the shooting before speaking at the White House with reporters about steps his administration is taking to combat the coronavirus.

"Our hearts break for them and their loved ones," the president said. "We send our condolences. We’ll be with them, and it’s a terrible thing, a terrible thing."

Update regarding the critical incident that occurred on the 4100 block of West State Street. There is no active threat; however, this scene is still an active. — Milwaukee Police (@MilwaukeePolice) February 26, 2020

The attack occurred at a sprawling complex that includes a mix of corporate offices and brewing facilities. At least 600 people work at the complex, which is widely known in the Milwaukee area as "Miller Valley," a reference to the Miller Brewing Co that is now part of Molson Coors.

Molson Coors CEO Gavin Hattersley called the shooter "an active brewery employee".

"Unfortunately, I am devastated to share that we lost five other members of our family in this tragic incident," he said in a statement.

"There are no words to express the deep sadness many of us are feeling right now."

Emergency vehicles are parked near the entrance to Molson Coors headquarters in Milwaukee, Wisconsin [Rick Wood/Milwaukee Journal Sentinel/USA TODAY Network/Reuters]

He said the office would be closed for the rest of the week and the brewery shuttered "for the time being" to give people time to cope.

James Boyles told the Milwaukee Journal Sentinel that his wife, Lasonya Ragdales, works at Molson Coors in the claims department.

She was texting from inside the building earlier on Wednesday and told her husband that there was an active shooter and she was locked in a room with a bunch of co-workers, the Journal Sentinel reported.

Before Wednesday's shooting, there had been three mass killings in the US in 2020, with 12 total victims. All were shootings. In 2019, there were 44 mass killings, with 224 total victims.