An undercover New York police detective and another man were found guilty on Tuesday of assault for being part of a group of motorcyclists who dragged a driver from his car in Upper Manhattan and beat him in front of his wife and baby daughter.
But a State Supreme Court judge in Manhattan acquitted the two men of the top charge they faced: first-degree gang assault.
The defendants chose to have the judge, Maxwell Wiley, decide their cases rather than a jury. On Tuesday morning, after a three-week trial, Justice Wiley found the detective, Wojciech Braszczok, 34, guilty of second-degree assault, coercion, riot and criminal mischief. But he rejected the most serious charges, gang assault and first-degree assault, saying the prosecution had not proved the detective shared the intent of the other attackers to hurt the victim.
The other defendant who was on trial, Robert Sims, 36, an electrician, was convicted of attempted gang assault, second-degree assault, coercion, riot and criminal possession of a weapon.
Justice Wiley allowed the two men to remain free on bail pending their sentencing hearings, which have been scheduled for early August. Detective Braszczok faces at least two years in prison, while Mr. Sims faces at least three and a half years.
Lt. John Grimpel, a Police Department spokesman, said Detective Braszczok had been “suspended pending termination,” meaning he would be fired.
Nine other bikers had pleaded guilty to second-degree assault for their roles in the attack, which took place on Sept. 29, 2013, on 178th Street near Wadsworth Avenue during a motorcycle rally that drew hundreds of bikers.
The Manhattan district attorney, Cyrus R. Vance Jr., said on Tuesday that the convictions showed his office’s resolve not to tolerate mob violence. “The explosive nature of this gang assault, much of it captured on camera, was shocking, and for the victim and his family, a terrifying ordeal,” Mr. Vance said.
The victim, Alexian Lien, and his wife, Rosalyn Ng, testified that dozens of motorcyclists first forced their car to stop on the Henry Hudson Parkway near 125th Street after getting into a minor dispute with them on the road. The couple’s 2-year-old daughter was in the back seat.
When bikers surrounded their car and threatened them, Mr. Lien said, he hit the accelerator, running over one of the motorcyclists, Edwin Mieses, and breaking the man’s spine. Scores of bikers gave chase and caught up to Mr. Lien’s car on 178th Street.
The attack was recorded by one motorcyclist’s helmet camera and on an iPad belonging to another biker. In one video, Detective Braszczok can be seen punching the back window of the car, then kicking the side panel twice, just before others dragged Mr. Lien out.
The video, along with a series of photographs, also show Mr. Sims first trying to force Ms. Ng from the car, then walking around the rear end and joining the attack on Mr. Lien, stomping him with his feet. Both men were identified through their helmets and distinctive motorcyclist clothing.
Detective Braszczok never touched Mr. Lien. He testified that he was only trying to stop the car from fleeing an accident and that he did not know the other bikers would attack the driver. He said he reflexively smashed the rear window with his hand after a projectile punched a softball-size hole in it — a projectile he thought had come from inside the car. In that moment, he said he became scared, and a few seconds later fled on his motorcycle, seeing the crowd pummel Mr. Lien as he left.
Outside court, the detective’s lawyer, John Arlia, said Justice Wiley had vindicated his client. “He didn’t intentionally cause any injury to Mr. Lien or to anyone,” he said. “He wishes to move on with his life.”
Before his arrest in 2013, the detective had spent several years as an undercover investigator, working first as a drug purchaser in narcotics investigations, then being assigned to infiltrate the Occupy Wall Street movement.
During the trial, the detective had tried to keep his face hidden, covering his head with a T-shirt when he came to and left court. But after the verdict, Detective Braszczok allowed photographers to take his picture. He declined to explain why.
Mr. Sims’s lawyer, Luther Williams, said he hoped to persuade Justice Wiley to give his client — who spent 16 months in jail before posting bail — the minimum sentence, given the circumstances. Though Mr. Sims did not testify, his lawyers argued that his actions were provoked by the sight of Mr. Lien’s car crushing a fellow motorcyclist.
“This is a case where there is bad behavior on behalf of a lot of folks,” Mr. Williams said. “I think in the heat of the moment they did some dumb things, but I think a lot of folks did a lot of dumb things.”
Justice Wiley did not explain his decision to acquit the men of gang assault, except to say he had carefully parsed the evidence about the intent each man had during the episode and then applied the law.