Former U.S. Marine Daniel Penny turned himself in to New York City police on Friday in connection with the chokehold death of Jordan Neely aboard a subway train.
Penny, 24, was placed under arrest for second-degree manslaughter and handcuffed. He appeared in court under police guard and did not enter a plea.
Assistant district attorney Joshua Steinglass said prosecutors conducted a "thorough investigation" that included interviews with eyewitnesses, 911 callers and responding officers before moving forward with the criminal charge.
Video showed Penny putting Neely in a chokehold on May 1 following outbursts from Neely on an F train. Several witnesses observed Neely making threats, Steinglass told the judge. 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, Steinglass noted.
Defense attorney Thomas Kenniff said Penny "has been fully cooperative throughout this process."
Kenniff told reporters that Penny "turned himself in here voluntarily and with the sort of dignity and integrity that is characteristic of his dignity of service to this grateful nation."
Attorneys for Penny said in a statement Friday, "We fully expect that Danny will be exonerated of all charges."
Penny's surrender came one day after the Manhattan district attorney's office confirmed that he would be arrested for second-degree manslaughter, for which the maximum penalty is 15 years in prison.
Neely family attorney Lennon Edwards is advocating for second-degree murder charges, saying Penny should have known Neely could die after seeing him struggle during the chokehold.
Penny "acted with indifference," Neely family attorney Donte Mills added at a news conference hours after Penny turned himself in. "And we can't let that stand."
"For everybody saying, 'I've been on the train and I've been afraid before, and I can't tell you what I would've done in that situation.' I'm gonna tell you -- ask how you can help," Mills said. "Please, don't attack. Don't choke, don't kill, don't take someone's life."
"We don't want anybody afraid on the subway," Mills said. "But we want people to look at those that may be there in that situation and say, 'Why?' And, 'How can I help them or make a difference?'"
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.
Some witnesses reportedly 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.
In an earlier statement, Penny's attorneys offered "condolences to those close to Mr. Neely" and claimed, "Mr. Neely began aggressively threatening Daniel," and that the Marine veteran and others "acted to protect themselves."
"Mr. Neely had a documented history of violent and erratic behavior, the apparent result of ongoing and untreated mental illness," said the statement from the law firm of Raiser and Kenniff. "When Mr. Neely began aggressively threatening Daniel Penny and the other passengers, Daniel, with the help of others, acted to protect themselves, until help arrived. Daniel never intended to harm Mr. Neely and could not have foreseen his untimely death."
Neely's death following the chokehold was ruled a homicide.
"The investigation thus far has included numerous witness interviews, careful review of photo and video footage, and discussions with the Medical Examiner's Office," Manhattan District Attorney Alvin Bragg said in a statement Friday. "Jordan Neely should still be alive today, and my thoughts continue to be with his family and loved ones as they mourn his loss during this extremely painful time."
The Rev. Al Sharpton in a statement Friday called the charges against Penny "just step one in justice."
"Let's not forget that there were three people restraining him, and it is vital that the two others are also held accountable for their actions," Sharpton said. "The justice system needs to send a clear, loud message that vigilantism has never been acceptable."
The judge on Friday approved releasing Penny on bond. Penny's attorney said the former Marine lives in New York City and is in college, pursuing a bachelor's degree in architecture.
Kenniff noted, "There is nothing less indicative of flight risk than someone voluntarily surrendering."
The district attorney's office decided to move forward with charges without first going to a grand jury. The case will still be presented to a grand jury in the coming days as prosecutors work to secure an indictment, prosecutors said Friday.
Penny is set to return to court on July 17.
ABC News' Morgan Winsor and Emily Shapiro contributed to this report.