WESTWEGO, LA. -- A 4-year-old boy in Louisiana, sitting in the back of a car with two younger siblings, fatally shot himself in the forehead with a gun he found while his mother and another adult smoked marijuana in the front seat, authorities said.

An autopsy Monday found that Jarion Walker's gunshot wound was self-inflicted, Jefferson Parish Sheriff Joseph Lopinto said at a news conference late Monday, The Times-Picayune/The New Orleans Advocate reported.

"It was a contact wound to the forehead, right above the right eyebrow," Jefferson Parish Coroner Gerry Cvitanovich said.

The shooting happened about 10:45 p.m. Saturday in the city of Westwego, a suburb of New Orleans. Jarion was sitting in the back seat with two siblings age 22 months and about 1, authorities said.

The adults, who admitted to smoking marijuana at the time, said they heard the shot, Lopinto said.

The little boy was rushed to a hospital, where he died of his injuries, the sheriff said.

No one has been arrested, though detectives will complete the investigation and turn the results over to the Jefferson Parish District Attorney's Office, Lopinto said.

"I don't think it was appropriate to make an arrest at this point," he said. "They just lost a child, and it certainly wasn't intentional, by any means."

The gun used in the shooting belonged to the child's father, authorities said. The boy is believed to have found the weapon under the seat, Lopinto said.

Since the child's death, the Sheriff's Office is cautioning gun owners to secure their firearms.

Guns should never be left inside vehicles, the sheriff said.

"These types of deaths can certainly be prevented," he said.