US neighbourhood watch volunteer George Zimmerman publicly accused police of covering up the beating of a homeless black man by the son of a white officer more than a year before he shot unarmed teenager Trayvon Martin, it emerged today.

The parents of a shot Florida teenager have accused police of leaking information about their son being suspended for marijuana and details about the fight in which he died that portrayed him as the aggressor.

The US police chief who has been bitterly criticised for not arresting a neighbourhood watch volunteer in the shooting death of an unarmed black teenager and the county prosecutor both left the case, with the chief saying that he is temporarily stepping down to let passions cool.