Four members of a Fort Washington family were found dead in their home yesterday, victims of what appeared to be a bizarre case of accidental carbon monoxide poisoning. A fifth family member survived after being revived by ambulance personnel.

Prince George's County police said that the five members of the Hardebeck family -- all adults -- were found dead or near death about 11:30 a.m. yesterday at their home at 3201 Calydon Ct. They were the sons, daughters and wife of Marvin C. Hardebeck, 51, a State Department official who had left earlier this week for an assignment in Grenada, police said.

Police spokesman Robert Law said that when police arrived they found the family's car, a cream-colored Chrysler, still running in the garage. Bodies were found on both floors of the two-story brick house. One member of the family was in the bathroom, two others appeared to have gone to bed for the night and a brother and sister appeared to have died in the recreation room.

The dead are Lydia Hardebeck, 50, who was deaf, and her sons, Kurt, 26, Gary, 20, and daughter Linda, 21. Karen Hardebeck, 23, who is mentally retarded, was found in an upstairs bedroom where a window was open. She was revived by ambulance personnel, taken to Prince George's General Hospital and later flown to University of Maryland Hospital in Baltimore for specialized treatment. She was listed in fair condition last night.

Police appeared fairly certain yesterday that the incident was was an accident. Neighbors, who say the family has lived in the area for about seven years, said there had been no signs of distress among family members. In fact, they said, Linda Hardebeck was planning to leave for vacation in the Bahamas this weekend.

Police detectives said that there was no sign of forced entry and that apparently all but one window in the house was closed. The garage, which is under the house, was also shut tight to the outside. Police believe that Lydia Hardebeck was the last to come home, may have forgotten to turn the car engine off and it may have run all night. With so little ventilation, police said, the odorless carbon monoxide vapors could have killed within minutes, even before exhaust fumes were noticed.

Linda Hardebeck's boyfriend, whom neighbors identified as Greg Fields, may have been the last to speak with family members when he called the house at about 9:30 Wednesday night, police said. When his calls were unanswered yesterday morning, he went to the house, saw a body lying on the floor in a downstairs room and broke a window. Police said an overpowering gaseous odor flowed through the broken window, prompting Fields and neighbors to call police and fire officials.

Stunned neighbors stood outside the house in the pouring rain yesterday watching police remove one covered body after another, including those of the family's dog and cat.

"It's a terrible shock," said Patricia Saxton, who lives across the street. Saxton said she talked with Lydia Hardebeck Wednesday afternoon. "She was happy as a songbird. This had to be an accident." She said that Lydia Hardebeck, though deaf, was a proficient lip reader and chatted with her neighbors regularly.

Lydia Hardebeck worked parttime in the cafeteria at the nearby Lord Baltimore Junior High School. When she did not appear for work at 10 a.m. yesterday, cafeteria manager Minnie Pasternak went to the house and knocked on the door several times, according to school Principal Eugene Kidwell. When Pasternak got back to the school, she received a call from Linda Hardebeck's friend Fields. Pasternak returned to the house to meet Fields.

According to Katie Stockdale, 19, a neighbor, Linda was employed by a local day-care center; Kurt, who recently returned from military duty, worked at Toys R Us in Springfield, and Gary was a student at the University of Maryland.

Neighbors seemed to have a special affinity for Karen, the young, retarded daughter, because she had worked so hard to overcome her handicap. Since 1983 Karen has worked at the Rustler's Steak House on Allentown Road, according to manager Emma Hurley.

According to Saxton, this is the second time bad news has caused Marvin Hardebeck to be summoned home. About two years ago, when Hardebeck was out of the country, Karen, who was attending a special school for the handicapped, was seriously injured when a school bus accidentally rolled into a group of students. But she recovered from those injuries and was often seen riding her bicycle around the neighborhood.

Yesterday as Katie Stockdale stood in the rain fighting back tears and trying to answer questions, she said, "The thing that hurts the most is that if it had been sunny out, all of those windows would've been open."