Last year, when an investigator from the sheriff’s office in Jackson County, Ga., interviewed a 13-year-old and his father about a possible online threat to “shoot up a middle school,” the teen denied responsibility.

I have to take you at your word, the investigator, Dan Miller, told him, “and I hope you’re being honest with me.”

Nearly 16 months later, the police say that the teenager, Colt Gray, who is now 14, killed four people and injured nine others at Apalachee High School in Winder, Ga., on Wednesday. It was the deadliest school shooting in the state’s history.

Could law enforcement officials have done more to prevent it?

The painful and inevitable question has frequently dogged the police after shootings. Experts say that most mass shooters display warning signs before becoming violent, and officials have often received tips, calls or reports about concerning behavior, sometimes long before someone picks up a weapon.

But law enforcement officers, at least under traditional police training, are limited in what they can do in response. If a crime has not been committed or a subject does not meet the criteria to be sent for an involuntary mental health evaluation, the case is often closed.

“It’s easy in hindsight to say, ‘Well, they should have done more,’” said Adam Winkler, a law professor and gun policy expert at the University of California, Los Angeles. “But how many times do police get these leads that people are saying things online that are intercepted as threats that don’t lead to that kind of violence?”