Aurora police said officers first responded to the Henry Pratt Company in the 400-block of South Highland Avenue just before 1:30 p.m. Police said two of the four first officers on the scene were fired upon immediately. More officers arrived as backup and were also fired upon.
ATF and FBI agents responded to the scene at approximately 2:37 p.m.
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Police said five officers were shot and injured. They are in stable condition Friday night, officials said. Police said they found five civilians dead inside the building.
A SWAT force entered the 29,000 square foot building and found the gunman, identified as 45-year-old Gary Martin.
Police said the officers engaged Martin in gunfire and he was killed. It was not immediately clear if Martin was killed by a police bullet or his own gun.
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"There are no words for the kind of evil that robs our neighbors of their hopes, their dreams, their futures," said Governor JB Pritzker, who also attended the press briefing.
WATCH: Aurora Mayor Richard Irvin speaks about deadly shooting
"Our hearts go out to the victims in this horrific tragedy. Our prayers to their families during what I'm sure will be a long and painful healing process. But we will heal. We will come together as one community and stand by those in pain from today's great loss. We will stand together with those officers shot in the line of duty. We will come together and heal as One Aurora," said Aurora Mayor Richard Irvin.
Martin was a veteran assemblyman at the century-old valve plant. Law enforcement sources told the ABC7 I-Team he was summoned to a meeting room where he was told he was being fired. He responded with a pistol, opening fire on the executives in the room, and then took his anger to the plant floor where dozens of his coworkers were still on the lines.
"Next thing you know, he went walking back in front and we heard more shots and that's when we just left the building, and he started opening up on the room and he was just shooting everybody," said John Probst, witness and survivor, who has worked at the plant for 40 years.
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There would have been approximately 30 people in the building at the time of the shooting, Probst said.
Police said in addition to the five people killed, only one other person was shot. A total of six people were taken to area hospitals for treatment with non-life threatening injuries, some of which may have been sustained as they tried to escape the shooter.
The I-Team has learned that Martin has a criminal background, and that he was convicted in 1995 for aggravated assault, a felony, in Mississippi and served two and a half years in the Mississippi Department of Corrections. It was not immediately known if he had a criminal record in Illinois.
There was a large law enforcement presence at Martin's apartment as investigators try to determine his motive and whether any part of this attack may have been planned. SWAT teams and a bomb squad swarmed the parking lot of the otherwise nondescript apartment complex.
"You never know who you're living with," said Patrick, a neighbor.
Technicians worked with a robot, sending it into the gunman's apartment to look for anything, perhaps hazardous, that might give them a clue as to what led up to the shooting.
Investigators are trying to piece together a clearer picture of who Martin is.
"He didn't seem odd," Patrick said. "He kind of kept to himself. Never really saw him with anybody. Just kind of strange."
"People are saying like he's weird. He wasn't weird. He's a good person," said Sherry Spears.
With tears running down her face, Spears described Martin - who she called Ted - as a kind, old friend.
"Just in school, we hung out in school. Knowing him, he left town and came back, but he ain't no bad person," she said.
While investigators search his belonging and electronics for clues, Spears offered a little insight.
"People are tired, depressed, so much going on. I wish the city of Aurora had something for people with mental health," she said.
Employees at Henry Pratt were kept on lockdown in another building well into the evening, but were allowed to leave around 8 p.m. after every corner of the facility was deemed secure by heavily-armed SWAT teams and K-9 units.
When they left, some carried their lunch cooler like any other day, and another employee carried Valentine's Day roses and a "happy birthday" gift bag.
"It cuts really deep, really deep. I have no words to express it, what the families are going through or what the officers are going through," said Rhonda Morris, lives nearby.
Mueller Water Products, the parent company of Henry Pratt, released a statement, saying:
Mueller Water Products is shocked and deeply saddened by the horrific tragedy that occurred today at our Henry Pratt facility in Aurora, Illinois. Our hearts are with the victims and their loved ones, the first responders, the Aurora community and the entire Mueller family during this extremely difficult time. Our entire focus is on the health and wellbeing of our colleagues, and we are committed to providing any and all support to them and their families. We continue to work closely with law enforcement, with whom we share our deepest gratitude for their support. We will provide updates as we learn more.
The sound of gunfire and police sires had neighbors too scared to go outside of their homes for most of the afternoon.
"It sounded like a machine gun," said a neighbor named Maria.
Residents were told to stay away from the active scene.
"Me and my husband were coming back from the store and I saw the first squad car pull up and then there were hundreds just going through my street. And I told my husband, I said, we have to go on lockdown," said Margie, neighbor.
West Aurora Schools were on soft lockdown for several hours. The lockdown was lifted at about 3:30 p.m. and students were dismissed. Parents tried to locate their children at different dismissal points.
Brianna, who attends West Aurora High School, worried for her safety.
"They didn't tell us anything," she said. "They just said that it was not a drill and that we couldn't leave."
The community is thankful for the first responders and heartbroken over the loss of the innocent victims.
"I've never seen this before," Margie said.
Probst said he and a coworker escaped through the back door. Probst said a nearby resident allowed Probst and his coworker to shelter in his home.
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Senator Dick Durbin tweeted Friday afternoon to express his gratitude for law enforcement officers who helped bring the shooter into custody.
My heart breaks for Aurora. I'm tracking updates on the situation with my staff. Thank you to the members of law enforcement who are responding to the emergency.— Senator Dick Durbin (@SenatorDurbin) February 15, 2019
Press Secretary Sarah Sanders said President Donald Trump has been briefed on the shooting.
Great job by law enforcement in Aurora, Illinois. Heartfelt condolences to all of the victims and their families. America is with you!— Donald J. Trump (@realDonaldTrump) February 15, 2019
The Illinois Fraternal Order of Police said in a statement released Friday:
"Every police officer dreads days like this one, yet these four courageous Aurora officers and their colleagues did not hesitate to literally put their lives on the line today to stop further bloodshed. These four heroes willingly ran into harm's way to protect their fellow citizens and very nearly paid the ultimate price. We Illinoisans should be humbly grateful for their sacrifice, and we ask that you join us in praying that the injured civilians and police officers make a full and speedy recovery."