A Fairfax County police officer rescued a teen-age girl trapped inside her burning house before dawn yesterday but was unable to save a 14-year-old Waldorf girl who was spending the night at the home and who died in the blaze, fire and police officials said.

Ojena R. Cowan, 13, of 2118 Arlington Ter., found unconscious near a second-story window by Officer Paul O'Keefe, was taken to Mount Vernon Hospital. She was airlifted by helicopter at midday to Children's Hospital in the District where she was reported in critical condition, suffering from a brain injury, lung damage and smoke inhalation, a hospital spokesman said.

Authorities said Cowan's friend, DeQuincy J. Hamlet of 6 Ferbane Ct., Waldorf, a La Plata High School ninth grader, died in the blaze that broke out minutes before 7 a.m. in the two-story duplex south of Alexandria.

County fire officials said the cause of the fire was under investigation.

Officials said O'Keefe was on a call at a nearby convenience store when he was alerted to the fire. He and a neighbor, Milton Jeffries, attempted to enter a second-floor bedroom at the rear of the house where Hamlet was later found, but were forced back by intense heat and smoke. O'Keefe, nearly overcome, fell down the stairs, according to Jeffries and police officials.

"We got down on our bellies but we couldn't get in there," said Jeffries, who said he pulled O'Keefe away from the blaze.

O'Keefe then got two bystanders to lift him onto the house's front porch roof. He broke the bedroom window, cutting his hand, and, with his head and shoulders inside the house, began to grope through thick smoke for other occupants, police said.

O'Keefe's supervisor, Sgt. Pat Gregoire, said O'Keefe located the Cowan girl on a bed and was holding her in his arms, unable to pull her onto the pitched porch roof, when fire trucks arrived. Gregoire said O'Keefe was enveloped in smoke and that fire personnel first realized where the officer was when they heard him cough.

O'Keefe, 27, was listed in good condition yesterday at Mount Vernon Hospital, suffering from facial burns, cuts and smoke inhalation. Reached by telephone, he declined to discuss the incident.

Also injured was Cowan's mother, Erma Cowan, 33, who police said was outside the house when rescue personnel arrived. The elder Cowan suffered burns on her face and was in fair condition at Mount Vernon Hospital.

Fairfax County Red Cross officials said a man identified as Josiah Cole, who occupied a basement apartment at the address, was uninjured and was given temporary lodging by the Red Cross at a motel on Rte. 1 south of Alexandria.

A county fire spokesman said damage from the fire, which was mostly confined to the rear of the house's second floor, was estimated at $35,000.