Daniel Penny has admitted he put a homeless man in a chokehold in a subway car on May 1, 2023.

Mr. Penny, a former Marine, said he believed the man, Jordan Neely, a 30-year-old Michael Jackson impersonator who ranted about being hungry and desperate, was poised to kill someone. He had to be stopped, Mr. Penny, then 24, told investigators, before he could hurt someone on the busy F train rumbling through Manhattan.

Mr. Penny’s actions, according to prosecutors, became criminal when he refused to let go of Mr. Neely long after he had gone limp. When Mr. Penny finally released his grip, a journalist who had filmed the chokehold kept his phone’s camera trained on Mr. Penny, who stood over Mr. Neely’s sprawled frame, staring down at him and not moving to help. Mr. Neely was pronounced dead that day at Lenox Health hospital in Greenwich Village.

On Monday, Mr. Penny’s trial on charges of manslaughter and criminally negligent homicide will begin in Manhattan Criminal Court. It could last six weeks.

The violent encounter divided the city, with some New Yorkers saying that Mr. Penny’s actions reflected transit riders’ fears and frustrations. Crime rates on the subway were higher at the time than they were before the Covid pandemic and there were regular reports of people being shoved onto the tracks or assaulted on station platforms.

But others said the killing showed the city’s inability or unwillingness to help its most vulnerable and marginalized residents. Mr. Penny, according to prosecutors, may have acted out of fear, but in the end he failed to see Mr. Neely’s humanity.

On Monday, the process of picking the jurors who will ultimately judge Mr. Penny’s actions will begin. Here is what to expect:

What evidence will prosecutors present?

Dafna Yoran and Jillian Shartrand, of the Manhattan district attorney’s office, will prosecute the case against Mr. Penny, which is expected to rely on a four-minute video that captured the violent encounter, as well as on Mr. Penny’s recorded statements to detectives in the hours that followed and the testimony of eyewitnesses.

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