A gunman opened fire inside the Henry Pratt Company in Aurora, Illinois on February 15, killing five people and injuring six others.

He was later identified as, Gary Martin, an employee of the firm, who had earlier threatened to kill everyone in the plant if he got fired. However, no one paid attention to his threats.

According to authorities, when 45-year-old Martin showed up for work on February 15, he told one employee: "If I get fired, I'm going to kill every mother*****r in here, and I am going to blow police up."

That employee, however, did not report the comments because he believed that Martin "was always making 'off the wall' statements," an official review of the shooting dated April 26 has revealed. The worker has not been named in the review, according to CNN.

Martin carried through with his threat that same day as he opened fire on the workers. He was among the five people who were killed in the incident as he was shot in a shootout. Five of the six injured were police officers.

It was later revealed that Martin should not have owned a gun because of a felony conviction in Mississippi, revealing gaps in federal background checks. 

A review of the incident was conducted by the Kane County State's Attorney office, which found that police officers were "wholly justified" in using deadly force during the mass shooting. The report was submitted to law enforcement on Friday and was publicly released on Monday.

State attorney Joseph McMahon, in the review, said that in addition to shooting at officers and SWAT team members, Martin did not respond to multiple verbal commands to surrender.

"Based upon a review of the investigations, all officers at the scene of this incident had the reasonable belief that the deadly force they employed was necessary to prevent death of great bodily harm to themselves and other individuals as they attempted to locate and arrest the offender," McMahon said in his conclusion.

The state attorney added that Martin had reportedly brought the gun and ammunition into the plant when he arrived for work on February 15, as there was no video evidence of him returning to his vehicle prior to the shooting.

The review stated that the same employee who had heard Martin make shooting threats also knew that the 45-year-old carried a gun in his car. However, the employee had never seen Martin with a gun inside the building and was not aware if he had brought in a gun on the day of the incident.

Martin had reportedly told multiple employees that he was concerned he might be fired because of an incident a day before concerning safety glasses. Martin, later that day at 1:00 pm, was called into a meeting in an upstairs office and he was spotted walking to his workstation, retrieving something and put on a hooded sweatshirt. He reportedly stopped in the restroom before entering the office. 

According to the review, the human resources manager Clayton Parks told Martin that they would start his termination process, it was then when he began cursing. The plant manager Josh Pinkard present in the room told Martin that "it's over." He responded with saying "Yeah, it's over" and opened fire, killing Parks and Pinkard and two other employees in the office, Russell Beyer and Trevor Wehner.