The New York Police Department has launched a probe into why internal affairs investigators were initially kept in the dark about an off-duty police officer who was suspected of drunken driving in a car crash that seriously injured a passenger, a spokesman said Sunday.

More than eight hours passed before Officer Rafael Casiano was given a blood-alcohol test. Court records show that by then, he only had trace amounts of alcohol in his system.

The NYPD's Internal Affairs Bureau "is investigating why there was an inordinate delay in it being notified after an off-duty member of the service was involved in a one-car accident," spokesman Paul Browne said.

Casiano, 44, was driving on a Bronx expressway at about 4:30 a.m. Friday when his car smashed into a center divider. The passenger was another off-duty officer, Keith Paul, who suffered a head wound and had to be hospitalized.

Police who responded to the scene smelled alcohol on Casiano and said he admitted he had been drinking, according to the court documents cited by the Daily News.

But the officer wasn't given a breath test until 1 p.m., the documents say. It showed his blood-alcohol level was between .032 and .029 — well below the .08 limit.

Despite the delay, internal affairs investigators, "after interviewing EMT and medical staff and obtaining a warrant for the subject's blood, made the determination that he was unfit for duty, arrested him for DWI and suspended him," Browne said.

Casiano was arraigned Friday night on charges of DWI and vehicular assault and freed on $5,000 bail. The name of his lawyer was not immediately available.

The mother of the hospitalized officer told the Daily News that he and Casiano are close friends who had been at a Christmas party for their Manhattan precinct before the accident. She said her son doesn't remember what happened.

"This is my Christmas miracle," she said. "I'm so glad he is going to be OK."