A 10-year-old boy is facing a murder charge for shooting a seven-month-old baby girl dead with a gun he found under a mattress. 

The boy, who due to his age has not been named, shot baby Kiyomi Pawnell-Parker in the head on Saturday afternoon inside a home in St Louis, Missouri, police said.

Officers rushed Kiyomi to a nearby hospital in their police cruiser, but the doctors were unable to save her.

The boy was taken into custody and charged with first-degree murder.

Kiyomi's 19-year-old father Ca'Marion Pawnell was also arrested and charged with second-degree murder, first-degree child endangerment resulting in death and two counts of endangering the welfare of a child by creating substantial risk.

Pawnell told police that he owned the gun and had stored it under the mattress in a bedroom at the home, according to a probable cause statement obtained by KSDK.

The 10-year-old knew where the gun was being stored and told police he had taken it out to play with it before, the charging document said.

He also admitted to shooting Kiyomi in the head, officers said in the affidavit. 

A seven-year-old child was at the home during the deadly shooting.

Officers have not revealed the relationship between Pawnell, who is being held without bond, and the other two children.

Kiyomi's heartbroken mother Janyla Parker has launched a GoFundMe campaign in effort to raise money for the funeral and other related expenses.

'At just 19 years old, I never imagined I would be planning my seven-month-old daughter’s funeral instead of watching her grow,' Parker wrote.

'My precious baby, Kiyomi Pawnell-Parker, was taken from me far too soon due to a senseless act of gun violence. No parent should ever have to experience this kind of heartbreak.'

She described the death of her daughter as an 'unimaginable loss' and thanked all those who donate for 'helping me give my beautiful baby the farewell she deserves.'

Pawnell was a graduate of East St Louis High School in Illinois and was preparing to attend college, his grieving sister revealed on social media.

The teen's family are heartbroken over the loss of Kiyomi and claim that Pawnell is also devastated by the tragedy. 

The shooting remains under investigation by homicide detectives and child abuse investigators.