Sheriff officers lead Isaiah Sweet into the Clayton County Courthouse in Elkader, Iowa Tuesday May 15, 2012 for his initial appearance. Sweet is being charged for killing his grandparents Richard and Janet Sweet of Manchester, Iowa. on Friday May 11. (AP/Dubuque Telegraph Herald/ Pool)

It’s rare for a teenager to be accused of killing a parent or guardian, but it happens, and when it does, experts say it often stems from disturbing circumstances.

Isaiah Sweet, 17, was charged last week in the fatal shootings of his grandparents, Richard and Janet Sweet of Manchester. Police haven’t offered a motive or given many details of what happened from the last time the couple was thought to be alive May 11 to when their bodies were found in their home May 13.

Three witnesses told police Sweet admitted killing his grandparents, according to the warrant for his arrest on first-degree murder charges.

Forensic psychologists said they couldn’t comment on Sweet’s case, but agreed to shed some light on why some teens resort to deadly violence.

There were an average of 250 cases a year from 2000 to 2007 involving adolescents ages 12 to 17 who killed a parent or guardian, said Robert Kinscherff, clinical/forensic psychologist and director of Forensic Studies at Massachusetts School of Professional Psychology in Boston. That is less than 2 percent of all homicides, according to FBI statistics.

Kinscherff, also an attorney, said juveniles who kill usually fit into one of three categories. About 90 percent are severely abused or maltreated, 4 percent to 5 percent are extremely mentally ill and 3 percent to 4 percent have a long history of antisocial behavior and are manipulative, calculated and cold-blooded.

Kinscherff said in the first category, the teen has suffered from extreme trauma. There is a pattern of abuse or they are exposed to “intimate partner violence” for a long time, such as the father abusing the mother and the teen becoming the mother’s “defender.”

Kinscherff said in these severe circumstances the families become isolated with no outside source for help and there is usually a pattern of substance abuse in the family. The deadly event occurs when there’s an escalation in violence and the teen feels he or the parent he’s protecting isn’t safe.

Mug shot of Isaiah Sweet, accused in the murder of his grandparents in Manchester.

Craig Shifrin, clinical/forensic psychologist in Springfield, Mo., said it’s rare that abused juveniles kill their abuser unless they are deeply disturbed or it’s a severe abusive situation. Most adolescents in situations like that run away or attempt suicide.

Kinscherff agreed, saying juveniles are more likely to be killed by a parent.

Adolescents in the second category suffer from extreme mental illness and usually haven’t received any effective mental health care, the psychologists say.

“These are kids who are acutely paranoid and they believe that parent is going to hurt them, so they’re acting in self-defense and they usually use a firearm,” Kinscherff said. “Almost all juvenile killings are with handguns because it’s usually what’s around.”

In the last category are teens with a long history of antisocial behavior, Kinscherff said. They are unemotional and callous, they break rules at home and school and are devoid of guilt and remorse.

“They see parents as being in their way … an obstacle to something they want,” Kinscherff said.

Kinscherff said a good example is a case he evaluated where a 15-year-old boy with a long history of conflict shot his parents execution-style because he became enraged when they wouldn’t give him money to go see his girlfriend. He then shot his sister to eliminate any witnesses, hid the bodies and then pretended to be distraught and helped the police search for the bodies.

“He was predatory and tried to throw suspicion off himself,” Kinscherff said. “He also fabricated some story and tried to claim maltreatment but that didn’t work.”

Shifrin and Kinscherff said one thing to keep in mind regarding adolescent behavior is that they react differently than adults in stressful or traumatic situations because their brains are not fully developed.

Kinscherff said what someone may describe as “partying” could be a way a teen tries to manage emotional stress.

“They are using whatever is available to them at the time,” Kinscherff said.

Shifrin said teens act more impulsively and make poor judgments because they only consider the moment and aren’t thinking about the future or consequences.

“They are less capable of identifying options, especially under stressful situations,” Kinscherff said. “Adolescents are more prone to volatile situations.”