Three Mississippi teenagers are accused of killing a 6-year-old boy after stealing the car he was left in.

The child, Kingston Frazier, was left alone in a Toyota Camry in a Kroger parking lot around 1:15 a.m. Thursday, authorities said.

The three suspects pulled into the parking lot in a separate car, the Hind's County Sheriff's Office said in a statement. One of them allegedly got into the Camry before both vehicles pulled away. It's not clear whether the driver of the Camry knew the child was in the car.

When Kingston's mother came out of the store, she alerted a nearby sheriff's deputy that her vehicle was missing. The Mississippi Bureau of Investigation issued an Amber Alert when it became clear that a child had been inside the stolen vehicle.

Kingston was found dead later Thursday morning in rural Madison County when a passerby noticed the car abandoned on the side of the road and recognized it from the Amber Alert. The boy was shot in the back of the head, Madison County Sheriff's Office spokesperson Heath Hall said.

Dwan Wakefield, D'Allen Washington and Byron McBride -- all in their late teens -- are being held without bond in the Madison County Detention Center, Hall said.

All three will be charged as adults with capital murder for the killing of a child during a kidnapping, District Attorney Michael Guest said in a press conference Thursday. They're also facing auto theft charges in Hinds County, according to the Hinds County Sheriff's Office.

Multiple state and local agencies collaborated on the investigation, including the Jackson Police Department, the Hinds County Sheriff's Office, the Madison County Sheriff's Office, the Mississippi Highway Patrol and the MBI. The FBI is also investigating, Hall said.