At Trolley Square, five people — Jeffrey Walker, 52; Vanessa Quinn, 29; Kirsten Hinckley, 15; Brad Frantz, 24; and Teresa Ellis, 29 — were killed. The study found women are more likely to be victims in a mass-murder spree (30.9 percent), while other studies have said men are much more likely to be victims of a general homicide.

The more complex issue is guns. Firearms were used 88 percent of the time in the study. Addressing gun control and assault-weapon bans, the study noted 63 percent of the guns used in mass-murder sprees were purchased legally and the buyer underwent a background check.

Salt Lake City police and the Bureau of Alcohol, Tobacco, Firearms and Explosives are still investigating how Talovic acquired his guns. Authorities are still looking into the Bosnian immigrant's background.

"Of the known offenders in this study, only 18.9 percent had any history of mental health problems," the study said. "Moreover, while a fairly high proportion of the offenders had a prior criminal history (50.9 percent), only 15.7 percent had a history of violent crime."

In the study, the most likely day for mass murder sprees was Monday, just like the Trolley Square shooting. Mondays accounted for nearly 22 percent of the incidents. Petee attributed that primarily to office-killing sprees.

"What happens with those, you get somebody that comes in and they had a previous bad week. They have a weekend to percolate and they come in on a Monday," he said.

The most frequent time of a killing rampage was between 9 a.m. and noon, suggesting many of the Monday morning mass murders were the result of workplace violence.

Even though they did not meet the study's definitions for a mass murder, the shooting rampages at a Chevy's restaurant in Midvale in 2000, the Triad Center in 1999 and the LDS Church's Family History Library in 1999 all occurred on a Thursday. That day tied with Tuesday for the second-highest day of the week for a mass murder. Saturday was the lowest at 8.1 percent.

"If more general categories were used for defining location, 40 percent of the mass-murder incidents occurred in a commercial location (i.e., restaurants, stores, etc.) and 31 percent in the workplace," the study said.

Breaking it down further, restaurants accounted for 16 percent of the sites for mass-murder sprees, followed by retail/grocery stores at 14.5 percent. Government offices were third, followed by schools. The study noted that a university campus was the most typical setting for school shootings.

Experts are not able to predict the next public mass murder, but Petee said he will continue to study the phenomenon. The Trolley Square rampage will become part of his research, he said.

"In general, this is a really rare event and your odds of becoming a victim of a mass murder are infinitely small," Petee said.

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E-mail: bwinslow@desnews.com