New York State's attorney general is investigating the March death of a black man after video footage emerged this week of him being hooded by police and forced face down in a road.

Daniel Prude's death became public after his family held a news conference Wednesday, as outrage mounts in the US over the police treatment of black people across the country.

Prude, 41, died a week after being detained, but relatives and other activists have called for legal action against the officers involved.

"The Special Investigations and Prosecutions Unit of my office is already actively investigating this incident," said a statement from Attorney General Letitia James. "We will work tirelessly to provide the transparency and accountability that all our communities deserve."

Prude's brother told reporters he called police to take his brother into custody on March 23 after he suffered a mental health episode. According to police body cam footage made public Wednesday, when officers arrived Prude was unarmed and naked in the road.

Police ordered an initially compliant Prude to get on the ground, but after being handcuffed he became increasingly agitated before officers put a "spit hood" on him and forced his head to the ground. He lost consciousness and officers tried to resuscitate him before he was taken to hospital, but he died a week later when life support was switched off.

According to the Rochester Democrat and Chronicle, an autopsy called the death a homicide caused by "complications of asphyxia in the setting of physical restraint."

The report also noted a low level of the drug PCP in Prude's blood, listing it as a complication. "He was treated inhumanely and without dignity," community organizer Ashley Gantt was quoted as saying by the newspaper. "These officers killed someone and are still patrolling in our community."

Since the death in May of George Floyd at the hands of a policeman in Minneapolis, several US cities have been rocked by protests under the umbrella of the Black Lives Matter movement. 

Opponents of the movement have also protested, frequently leading to violent clashes and pushing law and order high up the agenda for November's presidential election.