The heavily armed suspect killed three people and injured two others at a shopping centre near Indianapolis.
Elisjsha Dicken, 22, who was at the mall with his girlfriend, drew his pistol and fatally shot the gunman.
The incident has reignited America’s debate about gun ownership.
In the latest attack, a suspect opened fire with a rifle in a food court at the Greenwood Park Mall in the city of Greenwood.
The gunman had two rifles, a handgun and more than 100 rounds of bullets, said police.
He fired 24 rounds from one of the rifles before he was killed about two minutes into the assault.
Greenwood Police Chief Jim Ison said on Monday that Mr Dicken had engaged the gunman almost from the outset of the attack.
The police chief said that Mr Dicken had shown “proficient” and “sound” tactics in firing at “the gunman with a handgun from quite a distance”, despite having had no law enforcement or military training.
Mr Dicken also gestured for other shoppers to flee behind him “as he engaged and closed in on the suspect”, said the police chief. He fired 10 rounds.
Mr Dicken – who was lawfully carrying a concealed pistol – was at first handcuffed and questioned by police, before his version of events was confirmed by CCTV footage.
“Many more people would’ve died last night if not for a responsible armed citizen that took action very quickly within the first two minutes of the shooting,” Mr Ison said.
“I will say his actions were nothing short of heroic,” the police chief added.
The attacker was identified as a 20-year-old local resident who had recently been evicted and lost his job at a warehouse.
Two of the three people killed were a husband and wife, and the third was a 30-year-old man. A 22-year-old woman and a 12-year-old girl were injured.
Gun rights advocates held up the case as an example of why gun ownership by law-abiding citizens is important.
In a tweet on Monday morning, the National Rifle Association, a gun lobby, said: “We will say it again: The only way to stop a bad guy with a gun is a good guy with a gun.”
But such outcomes are unusual. Only two out of 61 such attacks last year ended when armed bystanders engaged the active shooter, according to an FBI report in May.
A series of recent mass shootings – including at a grocery store in Buffalo, New York, at a primary school in Uvalde, Texas, and at a parade in Highland Park, Illinois – have recharged America’s debate over gun laws.
Source: BBC