A grand jury has indicted former U.S. Marine Daniel Penny in connection with the chokehold death of Jordan Neely aboard a subway train, law enforcement sources told ABC News.

The exact charges will not be unsealed until Penny appears in court at a later date, the sources said. Penny was initially arrested on a second-degree manslaughter charge.

Video showed Penny, 24, putting Neely in a chokehold on May 1. Several witnesses observed Neely making threats, assistant district attorney Joshua Steinglass told the judge. Some witnesses told police that Neely was yelling and harassing passengers on the train, authorities said. Police sources told ABC News that Penny was not specifically being threatened by Neely when he intervened and that Neely had not become violent and had not been threatening anyone in particular.

Neely was homeless at the time of the incident.

Penny held Neely for several minutes, and at some point Neely stopped moving, but Penny continued to hold him for a period of time, Steinglass said. Penny remained on the scene to talk with police following the incident, Steinglass said.

The medical examiner determined Jordan Neely was killed by a chokehold and his death was ruled a homicide.

Steinglass said prosecutors conducted a "thorough investigation" that included interviews with eyewitnesses, 911 callers and responding officers before moving forward with the criminal charge. Penny turned himself in to police on May 12 following an announcement from the Manhattan district attorney's office regarding charges. He has not yet entered a plea.

Neely, who was homeless at the time of his death, had a documented mental health history, according to police sources. Neely had been previously arrested for several incidents on the subway, though it's unclear how many, if any, led to convictions.