Five employees were killed and five police officers were injured in the shooting. Martin was killed during the incident.
"Mueller Water Products is shocked and deeply saddened by the horrific tragedy that occurred today at our Henry Pratt facility," the company said in a tweet on Friday.
"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," the statement continued.
Police officers received an active-shooter alert Friday afternoon around 1:24 p.m. local time and arrived on the scene within four minutes.
All five police officers received "non life-threatening" injuries and were transported to local hospitals. Two of officers were later air-lifted to Chicago-area trauma centers. A sixth officer was being treated with a knee injury.
"Thank you for your selfless act," Aurora police chief Kristen Ziman said in a message to the law enforcement and first-responder community during a press conference. "Thank you for running towards gunfire and putting your lives in danger to protect those inside the business."
Here's what we know about Gary Martin:
Aurora police chief Kristen Ziman said they believe Martin was being let go from the company on Friday. Martin worked at the company for 15 years.
A woman who identified herself as Martins mother said he was "way too stressed out."
In 1994, Martin was convicted of stabbing a woman in Mississippi, according to court records. Six year later, he was found to have illegally modified his car speakers and received a notice for "squealing or screeching tires."
Martin lived in Aurora, but it was unclear if he was a life-long resident.
Two of four police officers who first responded to the incident and entered the building were fired upon immediately by Martin. He was reportedly armed with Smith & Wesson handgun. The initial gunfight lasted five minutes.
Preliminary evidence suggested Martin originally fired from a window. The entire duration of the incident lasted one hour and 35 minutes.
Police have yet to determine a motive and were looking at his criminal history.
