Stephen Naughton, the veteran Philadelphia police sergeant whose car plunged into the Schuylkill River yesterday afternoon, died of a heart attack, the Philadelphia Medical Examiner's Office ruled today.

Naughton, 55, died of acute myocardial infarction, with a secondary cause of death listed as drowning, office spokesman Jeff Moran said. The manner of death is listed as accidental.

Naughton, a married father of two and 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 lost control of his car, careened across the southbound lanes and into the river below.

An off-duty officer and a civilian who witnessed the accident dove into the water and tried fruitlessly to free Naughton from his seat belt. Another passer-by jumped into the river with a knife and cut him loose from his seat belt. Naughton, who worked in the Community Relations Unit, was pronounced dead at 4:45 p.m. at Hahnemann University Hospital.

Funeral arrangements have not been announced.