St Louis prosecutors refuse to charge man who gunned down unarmed black 13-year-old for stealing
A man aiming a handgun (Shutterstock)

A man who shot and killed a 13-year-old boy as he was stealing from an unlocked car was arrested but US prosecutors declined to press charges, police said.

Martinez Smith-Payne, who is black, was found unconscious and bleeding in a back alley when police were called about a shooting in St Louis, Missouri at 12:47 am Sunday. He died in hospital.

"Investigation revealed the victim, along with two other juveniles, was attempting to steal items from an unlocked parked vehicle when the suspect confronted them, then fired shots at the juveniles," St Louis police said in a statement.

The shooter, who was identified as a 60-year-old black man, was immediately taken into custody. He was released after prosecutors "refused" a police request to press charges, the statement said.

It was unclear if he was threatened by the teenagers prior to opening fire.

Police declined to provide further details about the incident.

Prosecutors called the boy's death a "a terrible tragedy" but said "Missouri law regarding a homeowner’s right to protect himself and his property is complicated."

Like many US states, Missouri's so-called castle doctrine allows for the use of lethal force without the "duty to retreat" if someone feels threatened. Such laws have been interpreted to also allow for the use of deadly force to defend property and have been used to attempt to justify shooting fleeing thieves, with varying degrees of success.

"We are reviewing the evidence, witness accounts and case law to ensure we are protecting the rights of both the victim and the homeowner," St Louis Circuit Attorney Jennifer Joyce said in a statement.

"All individuals deserve a thoughtful review of the facts."

The shooting comes as the region is still recovering from the police shooting of unarmed black teen Michael Brown in the St Louis suburb of Ferguson last year.

The shooting sparked months of sometimes violent protest and plunged the United States in a debate about racism and the use of force by law enforcement.

A boy with the same name was reported missing by his parents in June.

A local news report said Martinez Smith-Payne had attention deficit hyperactivity disorder and bipolar disorder and that his parents feared he had run away from home without his medications.

The boy, who lost four fingers in a fireworks accident at the age of 10, was found a few days later.