In the Bronx neighborhood where Eric Duprey lived, his fellow dirt bike and motorbike riders said he often rode down the street performing his signature move, with his front tire straight up in the air. But residents also said the area was a place where jobs were hard to come by, drugs were prevalent and encounters with the police could be fraught.

On Wednesday evening, Mr. Duprey, 30, was on a motorbike, fleeing narcotics officers who were attempting to arrest him, the police said, when a sergeant, Erik Duran, threw a cooler at him. Mr. Duprey lost control of the bike, hitting a tree and a car before the bike toppled over and he fell to the ground, a surveillance video reviewed by The New York Times shows. He was pronounced dead at the scene four minutes later.

On Friday, the New York City medical examiner’s office ruled Mr. Duprey’s death a homicide. The state attorney general’s office is investigating the incident.

Photos at a memorial for Mr. Duprey showed him with his children, Eryanis, 5, and Erian, 3.Credit…Stephanie Keith for The New York Times

The episode unfolded over just a few minutes around 5:30 p.m. on Wednesday. In the days since, Mr. Duprey’s family has disputed the Police Department’s account, and protesters have called for criminal charges against the sergeant who threw the cooler.

According to police officials, undercover narcotics officers were near 192nd Street and Aqueduct Avenue conducting a “buy-and-bust” — an operation to catch unsuspecting drug dealers in the area. During the operation, the police said, Mr. Duprey sold narcotics to one of the officers, which prompted undercover and plainclothes officers nearby to move in to arrest him.

A man, whom the police did not identify, rolled a motorbike over to Mr. Duprey, who got on and sped down Aqueduct Avenue, the police said.

Moments later, near the corner of 190th Street, the video shows Mr. Duprey driving onto the sidewalk and heading toward a group of people sitting at a table, which matches the police account.

It was then that Sergeant Duran grabbed a plastic cooler from the table and threw it at Mr. Duprey, an action captured by the surveillance camera.

Mr. Duprey’s brother, Ryan Rodriguez, said he and Mr. Duprey had been together on Wednesday, lingering on the sidewalk on the east side of Aqueduct Avenue.

“We were just here chilling and smoking, the same thing we do every day,” said Mr. Rodriguez, 21. Then, he said, a group of police officers approached Mr. Duprey while he was seated on a friend’s borrowed motorbike with the engine off.

Mr. Duprey started the bike and raced north in an attempt to outrun the police, Mr. Rodriguez said, adding that the reason that his brother had fled was that the bike was not registered.

“He was scared they would take his bike away,” Mr. Rodriguez said.

But Mr. Duprey found police officers at the next intersection, Mr. Rodriguez said. He whirled the bike around and drove south, on the sidewalk.

Sergeant Duran did not shout at Mr. Duprey to stop, Mr. Rodriguez said, adding, “He picked up the cooler and just smashed it on his face.”

When Mr. Duprey fell to the ground with the motorbike on top of him, Mr. Rodriguez said, Sergeant Duran ran over and lifted the bike from his body before performing CPR. But Mr. Duprey was clearly dead.

“It just broke my heart,” Mr. Rodriguez said.

Sergeant Duran was suspended without pay on Thursday, police officials said in a statement posted to social media. The Police Department said its force investigation division would help with the attorney general’s investigation.

“The N.Y.P.D. is committed to ensuring that there will be a full, thorough, transparent investigation of this incident to determine the facts and to take the appropriate steps forward,” the department said in a statement.

On Saturday, Jonathan Roberts, a lawyer representing Mr. Duprey’s family, said in a statement that Mr. Duprey had been “a loving partner, father and son who provided financial and emotional support to his family.”

He added that “nothing can mitigate this family’s pain, but the individual who committed this heinous act must be held accountable.”

Sergeant Duran’s lawyer, Andrew C. Quinn, said there was more to the situation than the video clip showed.

“What is indisputably clear is that the deceased, who was intent on evading arrest for selling drugs to an undercover officer, was speeding on a motorbike in an incredibly dangerous manner down a crowded sidewalk, jeopardizing the life and safety of everyone there,” Mr. Quinn said in a statement on Friday. “Once a full and complete investigation is conducted, we are fully confident that Sergeant Duran will be exonerated of any misconduct or wrongdoing.”

The swift suspension of Sergeant Duran, 35, a 13-year police veteran, provoked pushback within the department. One high-ranking officer said Sergeant Duran “had attempted to make a lawful arrest” and that using a cooler to stop Mr. Duprey was equivalent to “closing the door” to prevent someone from leaving.

The state attorney general’s office did not immediately respond to questions on Saturday about the possibility of criminal charges against Sergeant Duran.

It was unclear whether the sergeant’s actions violated rules laid out in the Police Department’s patrol guide. A department spokesman did not immediately respond to a request for comment on Saturday.

Because police officers face many unforeseeable circumstances in the field, the department does not always have specific guidance on what officers should and should not do in response to volatile situations or when they perceive a threat to themselves or others, said Maria Haberfeld, the chair of the Department of Law, Police Science and Criminal Justice Administration at John Jay College of Criminal Justice.

“This is not something that you’re going to see in the patrol guide, that you can pick up a cooler and throw it,” she said.

On Saturday afternoon, dozens of people joined activists for a small rally at the site of Mr. Duprey’s death. Many arrived on motorbikes, which they parked in a long row down the sidewalk.

Chivona Newsome, a founder of Black Lives Matter of Greater New York, called on the state attorney general to charge Sergeant Duran with murder.

“He needs to be charged the same way as if I threw a cooler at someone,” she said.

A day earlier, Mr. Rodriguez held solemn court at a memorial for Mr. Duprey, which included bouquets of flowers, bottles of Corona beer and tequila, many dozens of colorful votive candles, and blown-up color photos of Mr. Duprey with his wife and two young children.

“I want justice for my brother,” Mr. Rodriguez said. “I want that officer locked in jail.”

Chelsia Rose Marcius contributed reporting.