Illinois police are responding to a shooting at an industrial park in Aurora, a suburb about 40 miles (65km) from Chicago.
An unknown number of people have been injured in the attack, according to unconfirmed reports.
The Aurora Daily Herald says that police officers and several civilians have been injured.
A nearby school has been placed on lockdown. The FBI say they are responding to assist local police.
Around 15:00CST (21:00GMT), the City of Aurora tweeted that the suspect had been apprehended, but that the area is still on lockdown.
“Active Shooter Incident has been secured,” police tweeted, over an hour after the reports first emerged.
“Shooter is no longer a threat to the area. Continued police presence will remain as investigation continues.”
Several hospitals were put on notice, and were ready to begin accepting patients, according to local broadcaster WGN-TV.
The shooting is thought to have happened at Henry Pratt Company, a manufacturing company that makes valves.
An employee at nearby Capitol Printing told ABC7 that they were hiding in a closet.
Witness John Probst, who works at the plant, told 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, and the next thing you know he was walking back and forth, I heard more shots, and we just left the building,” Mr Probst told ABC7.
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 tweeted that she was monitoring the situation.
“This is a scary, sad day for all Illinoisans and Americans,” she wrote on Twitter.
Bill Donnell, an elected official in Aurora, told CNN that four police officers and several civilians were injured in the shooting, but this has not been confirmed by law enforcement.
The incident comes one day after the first anniversary of a school shooting in Parkland, Florida, which left 17 people dead.
President Donald Trump has been briefed on the incident, according to White House spokeswoman Sarah Sanders.