HOUSTON - Sexual abuse by his father and an increased dosage of Prozac may have helped drive a 10-year-old boy to shoot and kill his father last week, the boy's mother and attorney contend.

"My son is not a homicidal maniac," said Deborah Geisler. "I knew my son was angry with his father, but I never thought my son would see this as a way to handle the hopeless situation he thought he was in."

But the attorney who represented Dr. Rick Lohstroh during a bitter divorce denied the abuse allegations.

Lohstroh, a 41-year-old emergency-room doctor, was shot in the back Friday when he went to pick up his two sons at their mother's home.

Police say the 10-year-old boy climbed into the back of his father's sport utility vehicle, fired a pistol several times through the back of the driver's seat and then ran back inside the home.

Juvenile Court Judge Beverly Malazzo on Monday rejected a request from Geisler that he be released into her custody. The judge ordered that the boy, whose name is not being released because of his age, remain in a juvenile detention facility at least until a hearing on Sept. 13.

A psychiatrist prescribed Prozac for the boy in early August after he was diagnosed as suffering from depression and anxiety, his mother said Sunday. He started with a 10-milligram dose and gradually moved to higher doses, she said.

The week before the shooting, Geisler said, the boy started taking a once-a-week, time-release dosage of 90 milligrams. He took his second 90-milligram pill just hours before the shooting, she said.

The sexual abuse allegations could be central to the juvenile court case ahead, a lawyer for the 10-year-old said.

"I firmly believe this young man was a victim of physical and sexual abuse," said attorney Chris Tritico. "We are going to be doing some investigation, putting together evidence for the next detention hearing."

Tritico said he also is looking into the possible impact of the medication.

But Lohstroh's attorney, Kathleen Collins, adamantly denied allegations of abuse.

Lohstroh and his wife finalized a contentious divorce in May 2003 and had joint custody of the children. Geisler, 45, is a nurse.

Child Protective Services has a history of involvement with the family, but that information is confidential because the state never took custody of the children and no criminal charges were filed, said CPS spokeswoman Estella Olguin.

CPS reports are not public documents but would be made available to the judge who granted the parents joint custody.

Harris County sheriff's detectives are still investigating how the 10-year-old got the gun.

Geisler said the boy may have placed the Beretta semiautomatic pistol in his backpack before leaving the house to get into his father's vehicle.

The gun had been kept locked in its case, along with an ammunition clip, in a closet in her room, Geisler said. She said she thought it was a .40-caliber weapon, but she was not sure.

"It was stored, unloaded and locked," Geisler said. "The clip was out of it."

She said she bought it after her divorce because "it just made me feel safer." Her two sons were with her when she bought it, she said, but she did not know whether her older son knew where she kept it.

Assistant District Attorney Helen Jackson said she urged the juvenile court judge to keep the boy in a detention center because of the seriousness of the incident.

She said the district attorney's office will continue investigating to determine whether to file charges in juvenile court.

Under Texas law, children under 14 cannot be certified for trial as adults in criminal court.

Many juveniles are sentenced to incarceration with the Texas Youth Commission and released on their 18th birthdays. But even after reaching that point, Geisler's son could face up to 40 years in adult prison if he is convicted of murder and prosecutors seek to have him sentenced as an adult.