The “damning” video showing an off-duty cop waited just one second before gunning down a motorist in a Brooklyn road-rage incident is certain to result in criminal charges, law enforcement sources said Saturday.

Video of the July 4 incident obtained exclusively by The Post shows the victim, Delrawn Small, had just arrived at the window of Officer Wayne Isaacs’ car when Isaacs gunned him down.

Small, 37, is seen stumbling away from Isaacs’ car before he collapsed and died.

“The video is pretty damning,” a source close to the investigation said.

One police official believes a grand jury will be called in the case against Isaacs, 37, who could face a manslaughter charge.

“You’ll see some sort of charges filed,” the official said. “You have a person who is unarmed and they’re going to say the level of force used, versus force used against him, does not add up.

“When you use deadly force, there has to be some sort of grave danger to him. He can’t just say, ‘I got punched so I shot him.’ ”

Authorities are still looking for witnesses and additional video of the incident, which occurred early Monday on Atlantic Avenue and Bradford Street in East New York.

They want to talk to the driver of the car closest to the shooting, into which Small stumbled after he was shot. But the vehicle’s license plate is not readable on the video, sources said. “We’re really trying to track that person down,” the source said.

It’s unclear when charges would be brought, though one source speculated a grand jury could be convened as early as next week.

Prosecutors will want a solid case, according to retired NYPD sergeant and John Jay College of Criminal Justice professor Joseph Giacalone.

“Look what happened in Baltimore,” he said of the case against officers accused in the death of Freddie Gray, who died of injuries after being left unsecured in the back of a police van.

“If you go too fast, your whole case could be a disaster,” he said. “You better make sure you have your i’s dotted and your t’s crossed.”

The Small case is being handled by the state Attorney General’s Office, which issued a statement Friday vowing to “follow the facts and evidence — including this video evidence — wherever they lead.”

Sources said Small, who was in the car with his girlfriend, Zaquanna Albert, 35, thought the cop had cut him off on Atlantic Avenue.

‘You’ll see some sort of charges filed. You have a person who is unarmed and they’re going to say the level of force used, versus force used against him, does not add up.’ - an official to The Post

Small — an ex-con with 19 prior arrests — followed Isaacs’ car for several blocks and ignored Albert’s pleas for him to stay in the car.

Small walked across two lanes of traffic to confront Isaacs, who quickly shot him.

It was originally claimed that Isaacs, who was on his way home from a shift in the 79th Precinct, was pummeled repeatedly by Small before the cop opened fire. But the video refutes that idea.

The cop, who has since been placed on administrative leave, was bleeding inside his mouth, sources said.

“Deadly physical force is supposed to be used as a last resort,” Giacalone said. “[Isaacs] could have rolled up his window. He could have backed up his car . . .

“It doesn’t appear as if he was being physically assaulted, as he was claiming. So I think this video is going to be a problem for this officer.”

The charges could vary, lawyers not involved with the case said.

“If that video tells the entire story, I would venture to say that [prosecutors] must charge that cop with a minimum of aggravated assault and definitely manslaughter,” defense attorney Charles DeStefano said.

The officer could have avoided the incident, defense lawyer Ron Kuby said.

“After looking at the video on The Post’s website, what I’m seeing is an off-duty police officer who easily could have avoided the confrontation by rolling up his window and continuing to drive his vehicle,” he said.

“What I’m seeing is the police officer engaging in intentional unjustifiable killing of a human being, which is known as murder.”

Additional reporting by Jamie Schram, Dean Balsamini and ­Kathianne Boniello