The Manhattan district attorney confirmed on Thursday that Penny will be arrested on a charge of manslaughter.
The Manhattan District Attorney's Office spent the weekend and much of this week interviewing and going over the accounts of witnesses who were on the train, as well as reviewing multiple videos of the incident.
While more videos have been recovered, none show what happened before Penny put Neely in a chokehold.
Prosecutors also consulted with the medical examiner's office and detectives and reviewed statements Penny made to detectives on the night of the incident.
Taking all that into consideration, the district attorney's office decided to move forward with charges without first going to a grand jury.
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The second-degree manslaughter charge suggests prosecutors believe Penny had no intention to kill Neely and reflects a belief at the district attorney's office that Penny acted recklessly when he put Neely in a chokehold.
A grand jury will still hear evidence against Penny, which will occur in the week following his arraignment.
The maximum penalty for the charge is 15 years.
Neely, a homeless man, died after Penny allegedly held him in a chokehold for several minutes on May 1, according to witnesses and police. The 30-year-old's death has been ruled a homicide by the medical examiner's office.
Some witnesses reportedly told police that Neely was yelling and harassing passengers on the train before being subdued by the other passenger.
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According to police sources, Neely had a documented mental health history. Neely had been previously arrested for several incidents on the subway, though it's unclear how many, if any, led to convictions.
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.
Penny, a 24-year-old who served in the Marines, was questioned by detectives and released, according to police. He reportedly told police he was not trying to kill Neely.
In a statement, attorneys for Penny offered "condolences to those close to Mr. Neely" and claimed, "Mr. Neely began aggressively threatening Daniel," and that the Marine and others "acted to protect themselves."
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Protesters have called for justice following the deadly incident. Over the weekend, protesters filled subway stations, some jumping on the tracks, while pressing for more action. Several protesters have been arrested in recent days during demonstrations.
ABC News contributed to this report.
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