A security videotape shows an unarmed motorist exiting his car at a stoplight and leaning into the vehicle of an off-duty officer before the officer shot and killed him in an alleged road-rage incident.

The video, first released by the New York Post, has emerged as key evidence in a review by the New York Police Department and the state attorney general's office into the shooting of Delrawn Small early Monday.

Officer Wayne Issacs was placed on administrative leave pending the outcome of the investigation, police said. While investigators were certain the killing of Small was the result of a traffic dispute and suspect he accosted the patrolman, police Commissioner William Bratton has called it "much, much too early" to say whether it was justified.

Police said the incident unfolded shortly after Isaacs ended his shift at a Brooklyn precinct and drove away in his civilian clothes. It wasn't unusual that he was armed: The NYPD allows off-duty officers to carry weapons and to take police action if they encounter criminal activity.

Police say a woman who was riding with Small and their two children, ages 5 months and 14 years, told investigators that he became enraged after he thought the officer cut him off, then followed the officer's car to the stoplight despite her pleas to calm down and let it go. They say there was no was back and forth that would have made Isaacs aware that he was being pursued or that Small knew he was an officer.

The video appears to show Small walking around the front of the officer's car and leaning through the driver's window. Almost immediately, Small then stumbles back and ends up collapsing behind the officer's vehicle as it lurches forward.

The tape contradicts a news report quoting a man who claimed he saw Isaacs get out of his car and shoot Small. Investigators believe Small had punched Isaacs with the officer still seated behind the wheel before the officer fired three times, leaving behind shell cases found inside the car.

After the shooting, the woman in Small's car started to drive away with her children before returning to the scene, police said. Isaacs, who waited until patrol officers arrived, was taken to a hospital for treatment of minor injuries, they said.

State Assemblyman Charles Barron said Tuesday that he was working with Small's family to demand that authorities hold the officer accountable. The Brooklyn Democrat and frequent NYPD critic said the officer should have taken steps to defuse the situation with warnings or simply by driving away.

"Even if there was a punch, there's no justification -- zero -- for the use of deadly force," Barron said.

The police union that represents Isaacs declined to comment on Tuesday.