Police have taped off a suburban Chicago apartment unit where the suspect in the fatal shootings of six people in an industrial warehouse was believed to have lived.

Aurora, Illinois, Police Chief Kristen Ziman identified the gunman Friday as 45-year-old Gary Martin.

She said he was believed to have been an employee at Henry Pratt Co. Ziman said Martin killed five people before he was fatally shot by police.

Neighbours gathered on sidewalks near Martin's unit talking and wondering among themselves if they knew or had come in contact with the shooter. The multi-unit complex is a mix of apartments and condos.

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Mary McKnight stepped out of her car with a cherry cheesecake purchased for her son's birthday, to a flurry of police cars, officers and media trucks.

"This is a strange thing to come home to," she said.

Authorities responded to multiple casualties in Aurora, Illinois, west of Chicago, Friday afternoon, including several police officers, after a gunman opened fire inside a Henry Pratt warehouse. The suspect was "neutralised," according to a spokesman for the City of Aurora.

Chris Nelson, of the Kane County coroner's office, confirmed one fatality, though he wouldn't specify whether it was a victim or the shooter.

An Aurora police spokesman told The Washington Post that "multiple people ... multiple officers" were injured. Rush Copley, a community hospital in Aurora, said in a tweet that it received two patients involved in the shootings, and they were "being treated for non-life threatening injuries."

In an interview with ABC7, Aurora public information officer Clayton Muhammad said the injured officers were in "stable condition" and the suspect was "neutralised," though would not indicate whether the suspect was apprehended alive or dead. Clayton also told the station schools came off lock down around 3:45 Central time.

Police officers armed with rifles gather at the scene. Photo / AP
Police officers armed with rifles gather at the scene. Photo / AP

Tiffany Probst, 38, a legal assistant said her best friend saw a post on Facebook about the shooting and she started texting "that your dad might be inside!"

Her father, John, has worked as a machinist in the building for over 40 years. He has three grown children and has five grandchildren. Probst raced down to the factory but it was all blocked by police.

"I knew there was no way to call him because he's old school and never has a cellphone," then she heard from friends father was giving TV interviews and talking with the police.

"He's safe and talking to the news," she said. "He's not much of a talker, but when it comes to this, I can tell by his voice he's real shaken up. We are looking forward to giving him a hug."

Gabriel Gonzales, an Iraq War Marine veteran who can see the Henry Pratt warehouse from his front yard, said the number of police vehicles, flashing lights and armoured cars were giving him flashbacks.

"When you are a combat zone you expect it," said Gonzales. "I've never seen this many police officers anywhere."

He was watching his grandchildren, who were mesmerised by the activity unfolding through the window, and worrying about their brother Anthony, whose school was put under lock down.

"My grandson had a school lock down at 8 years old. I mean, can you believe that?" Gonzalez said. "Back when I was a kid, it was just tornadoes."

First responders and emergency vehicles are gathered near the scene. Photo / AP
First responders and emergency vehicles are gathered near the scene. Photo / AP

Sen. Tammy Duckworth, D-Ill., tweeted Friday that she was following the situation. "This is a scary, sad day for all Illinoisans and Americans," Duckworth wrote. "Thank you to the brave first responders who risked their lives this afternoon and apprehended the shooter."

"My heart breaks for Aurora," Sen. Richard Durbin, D-Ill., tweeted. "I'm tracking updates on the situation with my staff. Thank you to the members of law enforcement who are responding to the emergency."

Nancy Caal, who works at Erwin's Truck Repair near the scene of the shooting, told The Post that she heard the din of sirens as police cars and ambulances rushed to the building behind hers.

She and two others put the shop on lock down when they saw heavily armed officers heading toward the adjacent Henry Pratt warehouse.

"Nobody told us nothing," she said. "But we closed the gates and locked down the shop."

News reports of an active shooter there confirmed their fears shortly after.

"We are kind of nervous," Caal said. "It looks like something big is going on out there."

Additional details were not immediately available.