A man who jumped from a moving private ambulance, only to be fatally struck by a hit-and-run driver Monday evening in Holtsville, was suffering from mental illness, police said Tuesday.

Suffolk County acting Police Commissioner Tim Sini said at a Mineola news conference that the man, identified as Frank Ligrnetta, 51, of Bay Shore, was being transported from Stony Brook University Hospital to another hospital, when he unstrapped himself from a gurney. He opened the ambulance door and leapt out — only to be struck by a car on southbound Nicolls Road at 6:55 p.m., Sini said.

Ligrnetta was being taken to Brunswick Hospital Center in Amityville, police said. They believe that he was struck by the tire of the car and that the vehicle, whose driver fled, may not have sustained damage. But, Sini said, it was “unlikely” that the driver did not know he’d hit Ligrnetta.

VideoPatient jumps out of ambulance, struck and killed by car

Police said, at the time, the car, described only as a dark-colored sedan, was also headed south on Nicolls, south of Expressway Drive South.

Sini said, when Ligrnetta jumped out, two employees were in the ambulance, operated by Hunter, a private service — the driver and an attendant who was in the rear with Ligrnetta.

Sini said there was no struggle, that Ligrnetta simply managed to unfasten himself — and jump.

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“This is obviously a tragic event,” Sini said, “to have someone truly suffering from mental illness, in a very desperate place in their life, jumping out of the vehicle. Obviously, that person knew that death was likely, whether or not he was hit by a car . . . And then when you add the fact that it involved the hit-and-run, it just made the whole situation worse.”

The ambulance company did not return calls seeking comment.

Ligrnetta was pronounced dead at the scene.

The ambulance was impounded for a safety check, and Nicolls Road was closed for several hours for an investigation, police said.

Police ask anyone with more information to call the unit at 631-852-6555 or Suffolk Crime Stoppers at 800-220-8477. All calls will remain confidential.