A 55-year-old Philadelphia police sergeant died Tuesday afternoon when his car plunged into the Schuylkill, authorities said.

Sgt. Stephen Naughton, a 31-year veteran, had just left work at Police Headquarters and was headed to his home in the Andorra section of northwest Philadelphia.

Police Commissioner Charles H. Ramsey declined to name the officer, citing pending family notifications, but sources confirmed that it was Naughton, who worked in the Community Relations Unit.

As Naughton drove north on Kelly Drive shortly after 4 p.m., his light-colored sedan veered across the southbound lanes and into the water, police said. The accident occurred south of Hunting Park Avenue.

Another off-duty police officer driving south on Kelly Drive stopped and tried to rescue Naughton, officials said. She dived down several times but was unable to free Naughton from his seat belt.

A student from the Philadelphia College of Medicine riding a bicycle to Boathouse Row "saw what happened and immediately dived in," said Mayor Nutter, who spoke to the media after meeting with Naughton's family.

A third person arriving on the scene had a knife, which was finally used to cut Naughton free, Nutter said.

Naughton, who was reported by medics to be in cardiac arrest, was transported to Hahnemann University Hospital, where he was pronounced dead at 4:45 p.m.

Family members, including Naughton's wife and at least one of his two children, rushed to the hospital.

Nutter and other top city officials and police commanders gathered at the hospital to offer their condolences and support.

The medical student was being treated Tuesday evening at Hahnemann for foot injuries. Police do not know the identity of the person with the knife.

"We're very proud of them and thank them profusely, and the family does as well," Nutter said of the Good Samaritans. He said the city would officially recognize them at a later date.

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 and 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.

Duff jumped in twice but said he was unable to grab hold of anything. Duff, who is 6 feet, 5 inches tall, said he was able to stand on the roof of the car with his head still above the water.

Naughton eventually was pulled from the water and secured to a harness to raise him over a river wall, Duff said. Naughton then was attached to a defibrillator and was given cardiopulmonary resuscitation before being taken away.

Contact staff writer Robert Moran at 215-854-5983 or bmoran@phillynews.com.

Inquirer staff writer Mike Newall contributed to this article.