Aurora Police Chief Kristen Ziman said the shooter, identified as 45-year-old Gary Martin, was an employee at the sprawling industrial complex in Aurora, a Far West Chicago suburb, but did not say what may have motivated the attack.
The Chicago Sun-Times newspaper reported that a woman who identified herself to police as Martin’s mother told authorities her son had been laid off two weeks ago and was "stressed out" before Friday’s shooting.
"Today is a sad day in the city of Aurora," Mayor Richard Irvin told reporters. Aurora is about 40 miles (65 km) west of Chicago.
Ziman said police received multiple emergency calls from the manufacturing warehouse at 1:24 p.m. reporting an active shooter. The first officers arrived on the scene four minutes later and two of them were struck by gunfire, she said.
All told, five officers were shot and wounded, Ziman said, before the gunman was killed by police returning fire. A sixth officer suffered a knee injury.
Inside the building, five civilians were found deceased. None of them were identified.
The wounded police officers were in stable condition at a local hospital, Clayton Muhammad, spokesman for the City of Aurora, told the local ABC affiliate.
Witness John Probst told CNN that he saw the gunman, a co-worker, running down an aisle of the manufacturing facility with a pistol equipped with a laser sight. Probst told the network that he saw people bleeding.
Video on local media showed numerous police cars surrounding a large commercial building in Aurora, the ground covered in snow.
The bloodshed, marking the latest spasm of gun violence in a nation where mass shootings have become almost commonplace, came a day after the one-year anniversary of the massacre of 17 people by a gunman at Marjory Stoneman Douglas High School in Florida.
A spokeswoman for U.S. President Donald Trump said that he was aware of the incident.
"The President has been briefed and is monitoring the ongoing situation in Aurora, IL," Sarah Sanders said in a statement.