A motorist whose drunken speeding on the Long Island Expressway started a chain of crashes that jurors found led directly to a Nassau County police officer’s death was convicted Thursday of charges that could send him to prison for up to two decades.

While James Ryan, 28, of Oakdale, wasn’t driving the vehicle that struck and fatally injured Officer Joseph Olivieri Jr., jurors decided Ryan’s actions in the early morning of Oct. 18, 2012 made him responsible for the veteran highway patrolman’s line-of-duty death.

On day three of deliberations, the Mineola panel convicted Ryan of charges including aggravated criminally negligent homicide, second-degree manslaughter, vehicular manslaughter, third-degree assault and drunken driving.

The jury acquitted Ryan, who had worked at a Bohemia vitamin company, of the top count of aggravated vehicular homicide.

Ryan didn’t visibly react to the verdict, and kept his composure as court officers handcuffed him after Nassau County Court Judge Philip Grella ordered him remanded into custody.

Prosecutors Maureen McCormick and Michael Bushwack argued during the trial that Ryan turned the LIE into a drunken speedway, setting events in motion that ended with Olivieri’s lifeless body lying on the pavement.

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Ryan got drunk at a Manhattan lounge and headed home, speeding east on the LIE before crashing into a BMW near Exit 35 in North Hills, prosecutors said. He kept driving and, after slamming on his Toyota Camry’s brakes, was rear-ended by a Honda Civic driven by Edward Wilson, an off-duty NYPD detective.

Olivieri, responding to the accident call, crossed the expressway on foot to reach Ryan’s car but was struck and fatally injured by a Cadillac Escalade.

Prosecutors decided the actions of Escalade driver Francis Belizaire, 50, of Bay Shore, weren’t criminal, and he got immunity after testifying before the grand jury that indicted Ryan.

Amid tears, Ryan’s mother, Laura Ryan, handed her son’s winter coat to a court officer Thursday before authorities led the defendant away.

He nodded on the way out to his mother and father, Patrick Ryan, a now-retired Port Authority police sergeant, and didn’t comment later while getting into a Sheriff’s Department van.

Olivieri’s father left court without commenting, as did the Ryans.

Nassau District Attorney Madeline Singas said the verdict “isn’t a victory to be celebrated.”

“The Olivieri family will never get their son back. And the defendant’s life will be forever impacted by what happened,” she said.

Singas said the jury delivered a message that “when you drive drunk and when you drive recklessly, then you are criminally responsible for the outcome.”

Defense attorney Marc Gann said there “absolutely” would be an appeal, and instructions the jury received from the judge on the law regarding causation would figure prominently in it.

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During the trial, Gann and co-counsel Zeena Abdi had argued Belizaire was solely responsible for the officer’s death, and there were intervening causes that broke the chain of events that led to Olivieri’s death.

By their verdict, jurors disagreed.

“We felt there was causation, one thing led to another,” said juror William Gibney, 62, of Long Beach. “First it was the intoxication, then it was the reckless driving and then it was the first accident, then he didn’t pull to the right and he never got off the road. ... That caused the second accident, and according to laws, one thing led to another.”

A Nassau judge had dismissed the top charges against Ryan in 2013, finding Olivieri’s death solely attributable to Belizaire. But an appellate court later restored the full indictment.

Ryan is now facing a minimum of 3 1/2 years to a maximum of 20 years in prison on the aggravated criminally negligent homicide conviction. He had faced up to 25 years if convicted of the top count.

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But Gibney said the panel didn’t convict Ryan of aggravated vehicular homicide because jurors didn’t feel Wilson suffered a serious injury — which was an element of the charge.

The defense had conceded during the trial that Ryan had been driving drunk and caused the first crash. His blood-alcohol level was measured at 0.13 percent — well above the legal intoxication threshold of 0.08.

But the defense claimed Wilson caused the second crash while Ryan’s car was at a full stop, and Belizaire caused the third of what were three separate crashes.

Belizaire, a flagman for the Metropolitan Transportation Authority, testified that he slowed while approaching the accident scene, and didn’t see Olivieri in the road until the last second.

He said lights from police cars impeded his vision, and he was “blinded” by headlights from Wilson’s car, which had ended up facing the wrong way on the LIE.

Gibney said jurors felt Belizaire was partially to blame, “but he wasn’t a part of the charges.”

McCormick told jurors during closing arguments that they should follow the law and convict Ryan even if they believed someone else also was to blame for Olivieri’s death.

But the defense had argued that “the alcohol and DWI” in the case “blinded” the district attorney’s office’s view of what happened, and authorities didn’t even do an accident reconstruction.

Gann said Thursday that while the defense had argued at trial that Belizaire’s actions were among those that broke the chain of causation, legal instructions that went to the jury didn’t include a definition of the term “intervening act.”

“I do believe that the appellate courts are going to look at this and they’re going to change what happened here,” Gann said. “I don’t think we’ve heard the last of this case. I certainly hope that we haven’t. Because I do think the law needs to be clarified in this area.”

Nassau Acting Police Commissioner Thomas Krumpter said the verdict “brings justice” to the Olivieri family.

“But to be quite clear, the family and this department will be living with police Officer Olivieri’s death today, tomorrow and forever. We’ll never forget the ultimate sacrifice that police Officer Olivieri made,” he said.

Olivieri, 43, of Middle Island, was the father of two and had been with the Nassau police force for 13 years after five years with the NYPD. An LIE overpass is now named in his honor.