A 36-year-old government artist and his two young children were found dead in the family car yesterday inside the locked garage of their Alexandria home in what police called an apparent double homicide and suicide.

Investigators said they believed the man killed his children and himself with carbon monoxide.

A neighbor said the father, Anthony J. Ranfone of 17 W. Caton Ave. in the Del Ray section, had been involved in a dispute with his wife, Karen, from whom he was separated, over the custody of their daughter, Christina, 7, a pupil at Mount Vernon Elementary School, and son, David, 4.

According to the neighbor, a court hearing was held in the dispute on Dec. 22 and another was scheduled. The neighbor, Jerry L. Cooper, who lives two doors from the Ranfone house, said he last saw Ranfone and the two children two days later, on Christmas Eve.

Cooper said the children had just opened their Christmas presents, and Ranfone was wrapping a present for Christina and David to give to their mother when they visited her.

Cooper said that Karen Ranfone went to New Jersey, where her relatives live, in August. He said she took the children with her at the time of the separation, but that Anthony Ranfone went to New Jersey around Thanksgiving and brought the children back to Alexandria.

Cooper said he did not know the outcome of the Dec. 22 custody hearing and described Ranfone as a quiet man who "did not talk to that many people in the neighborhood."

Police said last night they did not know how long Ranfone and the two children had been dead. They said they went to the garage yesterday evening after being called by a neighbor who was uneasy about not seeing the Ranfones for some time.

Police were called at about 4:50 p.m., according to Lt. Harry Lane, a commander in the criminal investigation division. When they arrived, he said, they looked through a window of the locked garage and saw Ranfone inside his auto.

Police broke into the garage, Lane said, and found Ranfone in the driver's seat of the car. One of the children was in the front; the other was in the rear.

All three were dead. A hose connected to the auto's exhaust pipe led inside the vehicle, which was not running. Police said the bodies showed no signs of violence.

Lane said an autopsy is scheduled and the investigation is continuing. He declined to say whether a note was found.

One neighbor described Ranfone, believed employed by the Department of Energy, as a "very good" artist whose house was filled with his off-hours work.

They depicted a variety of subjects, she said, "some scenery, some animals, some birds . . . He just had a very fine sense of line."