The murder of a parent at the hands of a son or daughter is rare in this country, occurring 250 to 300 times a year, according to experts.

That amounts to about 1.5 percent of all killings.

The shooting deaths in Cockeysville over the weekend are even more unusual because the boy accused of the shooting, the victims' eldest son, is 15 and because he is also accused of killing his younger brothers.

"That's very, very rare," said Kathleen Heide, a criminology professor at the University of South Florida who has written two books on children killing parents.

More commonly, the killer has reached adulthood before killing one or both of his parents, she said.

Nicholas W. Browning is being held without bail on charges of first-degree murder in the deaths of his parents and two brothers. Police suspect him of shooting his father, John W. Browning, 45, after an argument and then shooting his mother, Tamara Browning, 44, and his brothers, Gregory, 14, and Benjamin, 11, who were asleep in their beds late Friday or early Saturday.

Their bodies were discovered Saturday night in their home in the Baltimore County neighborhood. Police say Nicholas Browning confessed to the shootings and said the killings followed an argument he had with his father, an attorney.

Browning, an honor student and athlete who was close to becoming an Eagle Scout, had no criminal record and no history of substance abuse or mental illness, said his former attorney, Steven D. Silverman, who represented him in the bail hearing.

Browning has pleaded not guilty, and Silverman said the youth denied the killings early in his questioning by police.

According to FBI data for 2005, of the 14,860 homicides committed that year for which an offender was identified, 241 involved the death of a parent at the hands of a son or daughter.

A parent killed a son or daughter 435 times that year. Husbands were deemed responsible for killing their wives 594 times, and wives killed their husbands in 135 cases. The largest relationship category was "acquaintances," who were responsible for 3,200 deaths.

Brothers and sisters were responsible for 121 deaths of siblings.

"The situations can vary tremendously from one case to the next," said Dr. Neil Blumberg, a longtime forensic psychiatrist who has testified for prosecutors and defense lawyers in homicide cases.

Teenagers face a lot of stress and aren't as equipped to deal with it as adults are, making them prone to impulsive acts, said Dr. Kenneth M. Rogers, a psychiatry professor at the University of Maryland School of Medicine.

"Their brains, in general, are still developing," he said.

It takes up to 30 hours of interviews with a young killer, and an extensive review of his medical history and the police report, before a therapist can determine the youth's motives, experts say. It can be even more challenging when the defendant is a teenager.

"The hardest part is when I've examined these teenagers. They're so protective they don't want to reveal anything," said Dr. Michael Arambula, a Texas forensic psychiatrist.

Arambula testified last month on behalf of Bryan Kim, who was 18 when he killed his parents near Spokane, Wash., stabbing his father, a respiratory therapist, and bludgeoning his mother, a high school math teacher.

Kim, who was convicted in the killings, had an explosive temper and bipolar disorder, relatives said.

"These are horrific cases," Arambula said.

Experts who study youth violence say such acts are almost always presaged by some trigger and that warning signs that are often overlooked.

"There's always a reason. It's never a random event," said Dr. John Lion, a Sheppard Pratt psychiatrist who is not involved in the Browning case and has not treated Nicholas Browning. "Murder is a complicated business."

dennis.obrien@baltsun.com