Kansas City, MO – Authorities are investigating how a convicted felon who Tulsa police called “one of Tulsa’s most dangerous criminals” managed to walk into a gun store, pass a background check, and legally purchase the weapon he would use to shoot three Kansas City police officers the next day.

Police said surveillance video captured 25-year-old convicted felon Marlin Mack on July 6 walking into J's Fish and Chicken Market at 54th and Prospect, where he fatally shot a graduate student who was working there, WDAF reported.

Mack was spotted by undercover officers at the Sky Vue motel on July 15, after a nine-day manhunt.

When he realized he’d be found, he reacted immediately.

“He just popped up all of the sudden and engaged the officer,” Kansas City Police Chief Rick Smith said.

Mack shot and wounded two Kansas City officers at the motel, and fled in a car. Police chased him to a home on 29th and Topping Avenue where a standoff began.

During the hours-long standoff, Mack shot another Kansas City police officer, WDAF reported. Police returned fire and Mack was killed in the gun battle.

Afterward, investigators discovered that Mack had purchased an AK-47 pistol the day before the shootout at The Armory KC.

Police learned that Mack, who had been in and out of prison for years, had passed the Federal Bureau of Investigation’s (FBI) background check for purchasing firearms despite the fact he was a convicted felon and should have immediately popped up in the database, WDAF reported.

Sources told WDAF that when The Armory KC ran Mack’s FBI background check, the system returned “Proceed to Transfer,” meaning that Mack wasn’t flagged in any of the databases used to vet gun buyers.

When he was 17, Mack used a MAC-10 pistol in a violent attack and robbery of a woman in front of her children in Tulsa, Oklahoma, and then tried to kill a witness who called the police.

“He needs to be warehoused until he is old and decrepit," Tulsa Police Sergeant Dave Walker said at the time. “He is matter of fact. He just sits there and has no conscience whatsoever, so those are the ones that will eventually kill somebody."

As a result, the convicted felon had a more powerful new gun with which to shoot three Kansas City officers the next day.

WDAF was referred to the FBI’s Criminal Justice Information Services Division in Clarksburg, West Virginia, by the Kansas City FBI field office for more information about how the background check could have failed.

The division responsible for the National Instant Criminal Background Check System told WDAF they needed more time to research the matter before responding.

The system, which was created by the Brady Act and launched by the FBI in 1998, has had a number of high-profile failures in recent years.

Dylann Roof, who committed the 2015 Charleston church terrorist attack that left nine people dead, was able to legally purchase the weapon he used because of a failure of the FBI’s background check system, according to The Washington Post.

In the Sutherland Springs church shooting on Nov. 5, 2017, the shooter’s domestic violence court-martial conviction should have prevented him from purchasing the AR-556 semi-automatic rifle he used to murder 26 people and wound 20 others.

However, the U.S. Air Force failed to register the conviction with the FBI’s database, and Devin Kelly had been able to legally purchase a total of four guns over four years, The New York Times reported.

The Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) was also investigating the failure of the background check system in Mack’s case, but told WDAF they could not comment because the investigation was ongoing.