People who feel marginalized, Tomb said, "have difficulty developing a mature moral sense because they just aren't around people, so they don't work through moral issues, or the people they're around are similar isolates. They don't run things through a moral filter before they act." In addition, marginalized teens often feel depressed and are more apt to be playing out their fantasies when they kill people, he said.

Some mass shootings in Utah's recent history have been linked to mental illness. These include:

• De Kieu Duy, a woman with a history of mental illness, who is charged with the fatal shooting of AT&T employee Anne Sleater and wounding Triad Center building manager Brent Wightman in 1999. She has been civilly committed to the Utah State Hospital after having been found mentally incompetent to stand trial. If her mental state improves, she could face a murder trial, because the statute of limitations does not run out on murder.

• Sergei Babarin, whose family said he suffered from schizophrenia, in 1999 shot and killed security guard Don Thomas and visitor Patricia Irene Frengs, as well as wounding three other people, at the LDS Church's Family History Library. Babarin was shot and killed by police.

But there are no clear-cut explanations for the behavior of other people involved in terrifying incidents that occurred here, although the perpetrators displayed behavior that could be linked to mental problems. These include:

• Richard Worthington, who in 1991 barged into Alta View Hospital with guns and dynamite seeking to kill a particular doctor. Worthington occupied the building for 18 hours and fatally shot nurse Karla Roth. He pleaded guilty, but his lawyers later tried to get the plea withdrawn due to mental illness. Worthington ultimately committed suicide in prison.

• Clifford Lynn Draper in 1994 took over the Salt Lake City downtown library and kept 10 people hostage for more than five hours. Draper did not kill anyone but was fatally shot by a police officer who managed to infiltrate the hostages. The officer shot Draper --> after it became clear Draper intended to start killing the hostages.

• Quinn Robert Martinez, who in 2000 fatally gunned down Chevys Fresh Mex restaurant manager Jason Rasmussen and patron Peter Berg, in addition to wounding three other people. Martinez had been on a methamphetamine binge for two weeks before the murders, according to his ex-girlfriend, but there was no information about mental illness. He currently is serving two prison terms of life without parole.

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E-mail: lindat@desnews.com; jarvik@desnews.com