The assailant lost his job of 15 years before he started shooting.
A gunman killed five people and injured several police officers after opening fire in a warehouse near Chicago.
Gary Martin was in the process of being fired from his job of 15 years, at Henry Pratt manufacturing, when he started shooting, authorities said.
Police shot Martin dead after responding to report of an active shooter at 2pm local time (8pm GMT).
Aurora police chief Kristen Ziman said Martin used a handgun to kill five of his colleagues and injure another, who has been taken to hospital with non-life-threatening injuries. The wounded police officers have been described as in a stable condition.
The shooting comes a day after the anniversary of the Parkland, Florida shooting at Marjory Stoneman Douglas High School where 17 people were killed.
Aurora Mayor Richard C Irvin said: “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.”
President Donald Trump tweeted: “Great job by law enforcement in Aurora, Illinois. Heartfelt condolences to all of the victims and their families. America is with you!”
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