CHICAGO (CBS)– Police Chief Kristen Ziman said gunman Gary Martin, 45, was not legally in possession of the firearm he used to open fire inside a manufacturing plant Friday.

Aurora police held a press conference Saturday morning to provide an update on the shooting at the Henry Pratt building that left five dead and six police officers wounded.

Police said Martin was previously sent a letter by state police stating he needed to voluntarily relinquish his firearm and FOID card.

Martin applied for a Concealed Carry License in March of 2014. During the finger printing and background process, police were alerted of Martin’s previous felony conviction of aggravated assault in Mississippi from 1995.

Now, police are investigating how Martin was still in possession of the firearm, even after police sent the warning.

A total of 12 people, including the gunman, were shot. The five wounded police officers, all men, were shot within five minutes of arriving on the scene, according to Ziman.

Martin was hiding in the back corner of the building, and officials said it appeared he was waiting for police.

The suspect, exchanged gunfire with police as soon as officers entered the 29,000 square-foot Henry Pratt building around 1:30 p.m. Feb. 15.

Ziman said Martin arrived at a termination meeting armed before he started shooting at employees in the meeting.

All the victims who died were employees. A sixth employee was being treated for non life-threatening injuries.

Martin lived in Aurora and was employed at the company, a subsidiary of Mueller Water Products, for 15 years. The Atlanta-based company manufactures water measurement products, according to its website.

In a statement released Friday night, the company said it was left “shocked and deeply saddened by the horrific tragedy.”

“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 reads. “Our entire focus is on the health and well-being of our colleagues, and we are committed to providing any and all support to them and their families.”

Ziman said police were notified about the shooting at 1:24 p.m. They arrived on the scene at 1:28 p.m. and were immediately fired upon.

Five officers were shot. A sixth suffered a knee injury.

On Friday in a press conference, Aurora Mayor Richard Irvin said the following:

“I don’t think I can be clearer in saying today is a sad day. We have seen similar situations around our nation. But to experience it first hand is even more painful. A shame these shootings are commonplace, a shame that someone would be so selfish to think he has the right to take an innocent life. We as a society cannot allow the heartless acts to become a spot on the 10 o’clock news.”

Gov. JB Pritzker also attended the press conference on Friday.

“There is no way to prepare for the pain of losing innocent people,” Pritzker said. “In the state that you were elected to lead, the state that you were elected to protect, there is no way to prepare. There are no words for the kind of evil that robs our neighbors of their lives.”

Two officers were shot in the leg; another in the hip; one in the neck and a fifth officers was hit in the neck. The injuries are all described an non life-threatening.