New York's attorney general has said a grand jury will be formed to investigate the death of Daniel Prude, an unarmed black man who suffocated after being restrained by police.
Mr Prude - who suffered from mental health issues - died after officers put him in a "spit hood", designed to protect police from detainees' saliva.
Protests have been held after footage of the incident in Rochester emerged.
Seven police officers have been suspended.
The 41-year-old died in March however his death has only just been reported.
Attorney General Letitia James said in a statement: "The Prude family and the Rochester community have been through great pain and anguish. My office will immediately move to empanel a grand jury as part of our exhaustive investigation into this matter."
The move has been welcomed by Rochester Mayor Lovely Warren and New York Governor Andrew Cuomo. But a spokeswoman for the Rochester Police Department declined to comment.
Mr Prude's brother, Joe, told the New York Times: "I am ecstatic about this. But right now I'm still waiting on seeing the indictment and them being prosecuted to the full extent of the law."
What happened to Daniel Prude?
Joe said he called police on 23 March as Daniel was showing acute mental health problems. When officers arrived, he had been running naked through the streets.
In body camera footage obtained from the police by Mr Prude's family, he can be seen lying on the ground as officers restrain him. While sitting on the road, he becomes agitated, alternately asking for money or a gun.
He began spitting on the street, but does not appear to offer any physical resistance, according to the footage. An officer says that Mr Prude told them he had Covid-19, and they place the spit hood on him.
One officer can be seen pressing down on Mr Prude's head with both hands, saying "stop spitting". Mr Prude stops moving and goes quiet, and officers note he feels cold.
Paramedics are called and Mr Prude is taken to hospital. His family took him off life support a week later.
The medical examiner ruled his death as a homicide caused by "complications of asphyxia in the setting of physical restraint", with intoxication by the drug PCP, a contributing factor.
Mayor Warren said the city police chief had failed to inform her of the case until the beginning of last month.
But police chief La'Ron Singletary denied that his department had been trying to keep the details out of public view, and Michael Mazzaeo, president of the Rochester Police Locust Club, said the officers had followed their training "step by step".
The officers were only disciplined after the footage was released, five months after Mr Prude's death. Protests in the city have taken place nightly since the release of the footage.
Mr Prude's death came two months before that of George Floyd, whose killing while in police custody sparked widespread outrage and incited national and international demonstrations against police brutality and racism.