Over the last two weeks, a Staten Island man named James received an unexpected FedEx delivery. Then a certified letter. His phone rang constantly with calls from people on the 13th floor of Police Headquarters in Lower Manhattan. A taxi was parked outside his home, waiting for him.

James did not open the FedEx, the mail or the taxi door.

No matter the entreaties, James said, he was not going to testify at the disciplinary trial of Police Officer Daniel Pantaleo, who is accused of recklessly using a chokehold that led to the death of Eric Garner. One of the most important witnesses to Mr. Garner’s death, James, who had twice given his account under oath in earlier proceedings, said this latest one trivialized the killing of a man. In fear of police retaliation, he said, he does not want his last name published.

“I’m done,” he said.

The unanswered calls, the unopened letters, might well stand for the dwindling urgency of coming to grips with a police encounter and a death that not long ago had galvanized the country.

Mr. Garner’s final moments, calling out “I can’t breathe” 11 times, were captured on videotape nearly five years ago and helped propel the Black Lives Matter movement. Mr. Garner, who was being arrested on suspicion of selling loosies, or single untaxed cigarettes, had objected to being handcuffed.