QUEENS — An NYPD detective is set to tell a grand jury this week why he fatally shot an unarmed National Guardsman he said was driving wildly on the Grand Central Parkway last fall, DNAInfo.com New York has learned.

Sources say Det. Hassan Hamdy, an Emergency Service Unit veteran, is nervous but determined to explain events that prompted him to fire a single bullet into Noel Polanco who, he claims, suddenly reached downward as though he may have been grabbing a gun after his car was stopped.

"This was not an ordinary stop," a source close to Hamdy said.

The shooting came at the end of a car chase in the early hours of Oct. 4, during which Polanco, who sources said was driving drunk, had to be forced to pull over to the parkway median by an NYPD van that was pursuing him with lights and sirens blaring.

Hamdy’s fate will ultimately rest on his credibility in convincing the 23-member panel that he believed he was about to be threatened with deadly force when he shot Polanco, 22. Cops are allowed to fire if they believe they, or someone else, is in imminent danger.

Last week, Diane DeFerrari, who was a passenger in the front seat of Polanco’s car, appeared before the grand jury to repeat her claim that her friend was not a threat to Hamdy when the officer fired, sources told “On The Inside.”

DeFerrari has publicly claimed that she saw Polanco’s hands on the steering wheel at the time of the shooting, but during interviews with police she said she was looking at Hamdy.

Vanessa Rodriguez, an off-duty NYPD officer, was asleep in the back seat and is expected to testify next week that she did not see the shooting. She is currently on desk duty following a previous shoplifting arrest.

Hamdy’s appearance will assuredly be the climax of nearly four weeks of evidence presented by the office of Queens District Attorney Richard Brown in the controversial shooting.

A father of two and former Marine, Hamdy could face a manslaughter or reckless manslaughter charge, law enforcement officials say.

Polanco, whose National Guard unit was based in Kingston, was posthumously promoted to sergeant at his funeral. His death touched off protests against police tactics.

The shooting occurred when an NYPD van filled with elite ESU officers was returning from The Bronx, where the officers had assisted in executing a warrant. They were heading to a similar job in Brooklyn when they were passed by Polanco’s speeding car.

Earlier that evening, Polanco drank several glasses of Hennessey cognac at an Astoria pub where Ferrari tended bar, law enforcement sources said.

He was driving Ferrari home, along with Rodriguez, when he was stopped.

The ESU cops say they pulled Polanco’s Honda over because they believed he was going to hit another car or crash his own. They put on their lights and sirens to stop him, but Polanco sped away. Ferrari later told reporters she virtually pleaded with Polanco to stop.

Detective Hamdy, who was in uniform, was the first cop out of the van. He yelled, “Police,” and demanded Polanco keep his hands on the wheel, the sources said.

Within seconds, however, Hamdy fired the fatal shot through the passenger window.

An autopsy later showed that Polanco’s blood alcohol level was .15 — nearly twice the legal limit, according to the sources.

Hamdy will try to make the grand jury understand that the shooting was the culmination of a “progression of events” that escalated after Polanco’s car whizzed past the cop van, the sources said.

Polanco’s family and lawyers claim the shooting is unjustified, regardless of whether he had some drinks, and that Hamdy panicked before firing.

Editor's Note: An earlier version of this story stated that Hassan Hamdy was expected to testify before the grand jury on Feb. 1. Subsequent information revealed that he could testify as early as Jan. 31.