Bizarre twist in case of toddler who "accidentally killed his 9-year-old brother".

When police were called to 28-year-old Wendy Lavarnia’s home, she told them her two-year-old son had accidentally shot his older brother.

She said she left the loaded gun on her bed at her Arizona home within reach of two of her children while she went to get a holster.

She heard a shot, and returned to find her nine-year-old son on the floor and her toddler holding the gun.

Landen Lavarnia was taken to hospital in a serious condition, but later died from his injuries.

Wendy and her husband, 31-year-old Kansas Lavernia, were arrested and faced child endangerment and gun possession charges after his death.

Now, in a bizarre new twist, police have launched a murder investigation into Landen’s death after they began to doubt the couple’s story.

According to Fox News, authorities believe the nine-year-old's parent's "put off calling 911" to clean up evidence in multiple rooms of their house.

A statement filed by police says the lack of visible blood and the extent of blood residue implied that "a significant amount of time passed before authorities were called".

"We have a nine-year-old critically wounded, shot in the head, in dire need of lifesaving efforts and care, which was delayed and not provided to this young man," police Sgt. Vince Lewis told Fox News.

"It definitely shocks your conscience."

The efforts taken to clean up the evidence have not been revealed by police.

Authorities also became suspicious when Landen's father, Kansas, showed up at the home three hours after the incident with a "crudely bandaged gunshot wound" on his upper arm.

Police believe the wound was punctured "multiple times, possibly with a screwdriver" to disguise the fact he had been shot.

Authorities have not yet named who they believe shot Landen and his father, but court documents say Kansas has been booked on suspicion of first-degree murder, child abuse and hindering prosecution.

Wendy Lavarnia has also been booked on suspicion of first-degree murder.

Charges have not yet been filed against the couple.

The couple's three surviving children are now in the care of a child welfare agency.