A employee of 15 years who was being fired from a suburban Chicago manufacturing company killed five co-workers and wounded five police officers, authorities said.
Police chief Kristen Ziman said that Gary Martin shot at officers as soon as they entered the Henry Pratt company building in Aurora - 40 miles west of Chicago.
Ms Ziman said that Martin, 45, was also killed.
She told a news conference that in addition to the five employees killed, a sixth worker was taken to a hospital with injuries that were not life threatening.
A sixth police officer suffered a knee injury while officers were searching the building.
Ms Ziman said officers arrived within four minutes of receiving reports of the shooting and were fired upon as soon as they entered the 29,000-square-foot manufacturing warehouse.
Police said they did not know the gunman's motive.
Great job by law enforcement in Aurora, Illinois. Heartfelt condolences to all of the victims and their families. America is with you!— Donald J. Trump (@realDonaldTrump) February 15, 2019
"May God bless the brave law enforcement officers who continue to run toward danger," Illinois governor J.B. Pritzker said at a news conference.
John Probst, an employee at the Henry Pratt Co. in Aurora, told ABC7 that he ran out of the back door as the shooting unfolded on Friday afternoon.
"What I saw was the guy running down the aisle with a pistol with a laser on it," he said.
Mr Probst said he was not hurt but that another colleague was "bleeding pretty bad".
"It's a shame that mass shootings such as this have become commonplace in our country. It's a shame that a cold and heartless offender would be so selfish as to think he has the right to take an innocent life," Aurora mayor Richard Irvin said.
The White House said president Donald Trump was briefed on the shooting and monitoring the situation as he prepared to depart for a weekend trip to his home in Palm Beach, Florida.
Mr Trump tweeted his thanks to law enforcement officers in Aurora and offered his condolences to the victims and their families. "America is with you," he said.