The gunman was also killed during an exchange of fire with police officers. Five officers were shot and wounded.
The shooting took place at a manufacturing company in Aurora, a suburb about 40 miles west of Chicago.
Police named the gunman as Gary Martin, 45, an employee at the company who was reportedly sacked prior to the attack.
Officers declined to speculate on a motive, but the Chicago Sun-Times newspaper is reporting that his family say he was "stressed out" by being made redundant.
The incident is said to have taken place at Henry Pratt Company, a firm that makes valves for large water pipes.
The gunman was also killed during an exchange of fire with police officers. Five officers were shot and wounded.
The shooting took place at a manufacturing company in Aurora, a suburb about 40 miles west of Chicago.
Police named the gunman as Gary Martin, 45, an employee at the company who was reportedly sacked prior to the attack.
Officers declined to speculate on a motive, but the Chicago Sun-Times newspaper is reporting that his family say he was "stressed out" by being made redundant.
The incident is said to have taken place at Henry Pratt Company, a firm that makes valves for large water pipes.
Chris Southwood of the Illinois Fraternal Order of Police described the Aurora officers who attended and were shot as "courageous".
"[These] officers and their colleagues did not hesitate to literally put their lives on the line today to stop further bloodshed," Mr Southwood said in a statement.
Of the five officers wounded, two were airlifted to nearby trauma centres.
The names of the five people who were killed have not yet been released.
How have witnesses described the scene?
Witness John Probst, who works at the Henry Pratt Company, earlier told broadcaster ABC7 that he saw the attacker, whom he recognised as a colleague.
He said the man was carrying a handgun equipped with a laser sight, but this has yet to be confirmed by officials.
"One of the guys was up in the office [and] he said this person was shooting, and, he come running down and he was bleeding pretty bad... I heard more shots, and we just left the building," Mr Probst said.
An employee at nearby Capitol Printing told ABC7 that they had hid in a closet when the shooting began.
Others in nearby buildings said they locked themselves in.
Police in Aurora, Colorado - where a man opened fire in a crowded movie theatre in 2012 - quickly tweeted that the incident was not happening in Colorado.
Illinois Senator Tammy Duckworth said: "This is a scary, sad day for all Illinoisans and Americans."
President Donald Trump posted a message on Twitter offering his condolences to the victims and their loved ones.
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