An off-duty NYPD cop waited just one second before ­fatally gunning down another driver in a Brooklyn road-rage incident early Monday.

Exclusive video obtained by The Post shows the moment that Officer Wayne Isaacs fired two shots through his car window at Delrawn Small, who collapsed and died in the street.

The NYPD and the state Attorney General’s Office are investigating the shooting, which the victim’s family said was a clear case of excessive force.

The footage, from Atlantic Avenue and Bradford Street in East New York just after midnight, shows Small, 37, cross two lanes of traffic to reach Isaacs’ car.

He barely has time to look the cop in the eye or even utter a word before Isaacs opens fire, causing him to stagger back.

He stumbles to the ground, gets up for a moment and then collapses again for good.

Isaacs, meanwhile, lurches his car forward a few feet before slamming on the brakes and getting out. He appears to tuck the gun into his waistband as he walks over toward Small.

Isaacs looks in the direction of the dying man, pausing for a few moments near his body, before returning to his vehicle.

He is then seen pacing around and talking on the phone. Sources have said he called 911.

Small’s girlfriend, Zaquanna Albert, 35, then pulls the man’s car across the street before running toward the scene.

Then the footage cuts off.

Police sources said Small’s temper flared when he thought Isaacs cut him off on Atlantic Avenue. Small, an ex-con with 19 prior arrests, followed the cop, who was in an unmarked vehicle, for several blocks ­before getting out at a traffic light to confront him.

Albert told investigators that she begged Small, who’d had three drinks at a barbecue they had just left, not to get out.

Isaacs, who has since been put on administrative duty, was on his way home after a shift in the 79th Precinct in Bedford-Stuyvesant. He used his service weapon to shoot Small in the head and chest, sources said.

Small’s brother, Victor Dempsey, said the video shows he was “point-blank murdered.”

“Now that I saw that video, I’m outraged,” he said at a press conference Friday night at the shooting scene. “It’s time for us to get justice on it. Everything they told us from the very ­beginning is a lie.”

Victoria Davis, Small’s sister, said watching the video was difficult. “Him stumbling, and even when the officer got out of the car, he didn’t seem to have any care,” she said.

“He just put his gun away.”

Brooklyn Assemblyman Charles Barron insisted “things can get out of control” if the attorney general doesn’t bring charges against the cop.

“We won’t have any next steps to tell our people to even bother with this system,” he said. “People will take matters into their own hands because they won’t have any other alternative.”

The family’s lawyer, Roger Wareham, said the video ­reveals “there was no threat to the cop. Deadly force was not justified.”

The video contradicts claims of the owner of a nearby building who said Small was seen on the video “punching the s–t” out of Isaacs after breaking free from Albert’s grasp.

Attorney General Eric Schneiderman promised to “follow the facts and evidence — including this video evidence — wherever they lead.”

The NYPD declined to comment.

Additional reporting by Joe Marino