Stephen Naughton, 55, a veteran Philadelphia police sergeant and married father of two, died this afternoon when his car plunged into the Schuylkill, police said.

The 31-year veteran of the force had just left work at Police Headquarters and was headed to his home in the Andorra section of northwest Philadelphia.

As he drove up Kelly Drive near Hunting Park Avenue shortly after 4 p.m., the officer somehow lost control of his car, police said. The vehicle careened across the southbound lanes and into the water below.

An off-duty officer who witnessed the accident dove into the water and tried several times in vain to free Naughton from his seat belt.

Another witness, a student from the Philadelphia College of Medicine, also saw what happened and dove into the river, authorities said.

The pair was unable to pull Naughton out of his Chrysler sedan until a until a third person jumped into the river with a knife and cut him loose.

Naughton, who worked in the Community Relations Unit, was pronounced dead at 4:45 p.m. at Hahnemann University Hospital. The cause of death was not immediately clear.

The medical student was being treated at Hahnemann last night for cuts and bruises. Police did not know the identity of the person with the knife.

Mayor Nutter, Police Commissioner Charles Ramsey and other top brass met with Naughton's family at the hospital.

"We were very, very saddened by the events of this afternoon," Nutter said. "It's always tragic when we lose any of our officers, whether on duty or off duty."

Nutter praised the Good Samaritans who tried to save Naughton and said the city would honor their efforts later.

Shawn Duff, 25, an assistant rowing coach at St. Joseph's Preparatory School, was at a boathouse near the Strawberry Mansion Bridge when he was told about the accident. He took a motor boat to the scene.

"There were three or four in the river bobbing up and down," Duff said, describing the scene of rescuers repeatedly diving to reach Naughton.

Naughton eventually was pulled from the water and secured to a harness to raise him over a river wall, Duff said.

The veteran cop then was attached to a defibrillator and was given cardiopulmonary resuscitation before being taken away.

McNesby said Naughton helped coordinate police-merit ceremonies that are frequently held at the FOP's headquarters on Spring Garden Street near Broad.

"He was a pure gentleman," said John McNesby, president of the Fraternal Order of Police Lodge 5.

"He never let anything slip by," McNesby said. "He always wanted to make sure the cops were recognized for everything they did.

"He also used to help us plan the details for funerals for our fallen officers. Now we're going to be planning his."

A steady stream of tearful relatives and friends shared their grief later in the evening at Naughton's home on Scotia Road.

Evelyn Bose, an 86-year-old widow who lives next door to Naughton's family, peered through her front window at parked police cruisers.

"This is too much of a shock," she said, as she wiped tears from her eyes.

Bose lived next-door to Naughton for about 20 years and remembered him as a caring, dependable neighbor who often invited her to family barbecues and other gatherings.

"He took care of me; he'd send meals over to me. He was always there for me if I needed him.

"I just can't begin to conceive that this is it, you know?"