Aurora investigators determined immediately after Friday’s killings of five people at a manufacturing firm that the shooter had worked there for 15 years. They soon learned he had caused the carnage with a handgun purchased from a local dealer in 2014. What’s now under question, as five families grieve and wounded police officers recuperate, is why the killer still had a gun he shouldn’t have possessed.

According to authorities, the man who fatally shot five employees of Henry Pratt Co. and wounded several officers Friday was a convicted felon. Gary Martin served time in prison in Mississippi in the 1990s for aggravated assault. Yet about a decade later, he was issued an Illinois firearm owner’s identification card and purchased a Smith & Wesson .40-caliber handgun.

Not everyone is allowed to have a FOID. A felony conviction is a disqualifier. Martin wasn’t flagged until after his purchase, when he sought a concealed carry license. It appears that during the background check process, which included fingerprinting, his criminal record popped up. What happened next? Frustratingly, not enough. Martin’s FOID was revoked, but he kept his firearm — later using it to cause a bloodbath in Aurora.

There are gun laws on the books designed to keep weapons away from dangerous people. Then there is enforcement. In this instance, there was a deadly lapse. Martin should never have received a FOID. Once that error was discovered, Illinois State Police apparently would have notified Martin by letter that he was not legally eligible to own a gun. Obviously, he either never got notification or ignored it. And no one took his gun away.