(CNN) The gunman who killed five people in an Illinois manufacturing plant pleaded guilty in 1995 to aggravated assault in Mississippi, a felony that should have prevented him from getting a gun.

But when Gary M. Martin applied for a gun owner's identification card in Illinois in 2014, he answered "no" to the question, "Have you ever been convicted of a felony?" Illinois State Police, or ISP, said Monday.

So began a series of apparent oversights that let Martin purchase the handgun he used in the February 15 rampage at Henry Pratt Co. in Aurora.

And later, when Illinois authorities caught the error, it appears nothing was done to get the weapon back from Martin.

Last week, Martin brought the gun to a meeting at Henry Pratt Co., where he'd worked for 15 years. When he found out he'd been fired, he shot the people in the room with him, authorities said. He then stormed into the warehouse, witnesses said, in a rampage before police killed him in a shootout. Martin killed five people and injured six people -- including five officers.

State police and other law enforcement agencies are now under pressure to explain how, despite his felony conviction, Martin slipped through background checks and was able to keep his gun.

Where the conviction comes from

Martin pleaded guilty in February 1995 in Mississippi to assaulting the woman he lived with.

The woman told police he assaulted her at least three times before that attack, according to court documents. In the March 1994 incident, he stabbed her with a knife and beat her with a baseball bat, an indictment states.

He was sentenced to 10 years and the court suspended five years of his sentence, according to court records. He was reportedly released on April 18, 1997, according to ISP.

The first background check did not catch the Mississippi conviction

After he left prison he moved to Aurora, where he had spent time as a child, according to court documents. He started working for Henry Pratt Co.

On January 17, 2014, Martin applied for a Firearm Owner's ID Card, or FOID card , through Illinois State Police. The application did not require him to submit his fingerprints.

It should have been denied. ISP's website lists disqualifying factors for owning a gun, including a felony conviction or a conviction in the past five years for violent crimes including aggravated assault or battery, among other criteria.

FOID staff search for applicants' names in state and federal databases to determine if they have a criminal record or other violations that make them ineligible to own a firearm, according to Lt. Joseph W. Hutchins of the Illinois State Police

The Chicago Tribune reported in 2016 that ISP uses the National Instant Criminal Background Check System, or NICS, a database maintained by the FBI that several states use. ISP could not confirm on Monday which databases from outside the state it tends to use.

But when FOID staff conducted a records search, it only produced Martin's criminal history in Illinois, "which revealed no prohibiting factors," ISP said in a statement.

Hutchins said ISP is still investigating why the Mississippi conviction did not come up.

The gunman bought the murder weapon

After Martin's FOID card application was approved on January 31, 2014, he purchased the .40-caliber Smith & Wesson used in the shooting.

The transaction was approved on March 6 "after the name and date of birth background check was again cleared," the ISP statement said.

Then, he got caught while applying for a concealed carry permit

Next, Martin applied for a firearms concealed carry license, another form of identification that ISP issues so gun owners can carry firearms.

ISP received Martin's application on March 16 with his fingerprints, the department said, noting that if fingerprints are included, processing time reduces from 120 days to 90.

The fingerprint background check produced an FBI number that led to a Mississippi Department of Corrections entry noting a charge of aggravated assault and a period of imprisonment, ISP said.

Martin's application was denied on March 26.

The agency revoked his FOID card ...

A letter dated April 15 notified him of the denial and told him was no longer eligible for a FOID card, which was revoked. And, in the letter, he was told he was responsible for surrendering his FOID card and any weapons in possession within 48 hours.

ISP's procedure in such cases at the time was to notify local law enforcement where the FOID card holder lives through the Illinois Law Enforcement Agencies Data System.

But the Aurora Police Department said no record suggests that Illinois State Police notified them of the revocation.

Meanwhile, Martin should have received a Firearm Disposition Record to document his relinquishment of the card and gun to law enforcement or someone else with a valid FOID card.

... but never got back his card or the weapon

But ISP says it has no such documentation related to Martin.

"The ISP has no record of receiving a Firearm Disposition Record for Martin or Martin's FOID card at this time, however a review of paper and electronic files continues," according to the agency's statement.

If the holder of a revoked card fails to comply, the county sheriff or law enforcement agency may petition the court for a search warrant for the FOID card and any firearms in the person's possession, ISP said. But Illinois law does not require them to do so.

"Any agency that receives notice of a person's FOID card revocation, they're not bound to go take action based on that revocation notice," Aurora Police Public Information Officer Bill Rowley said in a news conference on Tuesday.

"But we don't have any record that we received notice in the first place."

In 2018 alone, ISP said it issued 10,818 revocations to FOID card holders. In most cases, ISP said it does not receive any documentation in return noting if a gun or a FOID card has been relinquished.