The shooter was identified as a 51-year-old man who worked for the company, the home of Miller Brewery for more than a century.
All of the shooting victims died. There were no reports of injuries.
The identities of the victims and the shooter were not released Wednesday. Police did say, however, that the victims' families have been notified.
Milwaukee Police Chief Alfonso Morales said at an early evening news conference that police were checking on the whereabouts of more than 1,000 employees. "We have to check off employees one by one," he said.
By 9:30 p.m., that had been done and all employees at the brewery had been allowed to go home, Morales said at a news conference.
"Milwaukee is grieving today," said the police chief.
Milwaukee Mayor Tom Barrett noted at the late evening news conference that the brewery has been a landmark in Miller Valley for 165 years. "It's the saddest day we have had" in that history, he said.
Comment: UPDATE February 28
The gunman has been identified as 51-year-old Anthony Ferrill, who was recently fired by the company and reportedly had quarreled with one of the electricians before the shooting.
A former co-worker claimed Ferrill had been showing signs of paranoia - he "believed he was being discriminated against because he was African American," and that he later became convinced that "brewery workers were coming into his home" and "bugging his computer."
Police and city officials have declined to say whether the shooting was racially-motivated as the investigation is in preliminary stages.