An Arkansas man apparently kills his family, then goes on a shooting spree. An Iowa farm family is wiped out; the son is suspected of murder-suicide. A postal employe kills 14 coworkers in Oklahoma, then kills himself.

Are these cases isolated, or is the phenomenon of mass murder growing?

Experts say the recent spate of slayings does not signal an epidemic.

However, they warn that factors such as an aging baby boom generation and a growing rootlessness may yield a crop of multiple killers.

"Mass murder is still a rare phenomenon and it's hard to make predictions," said James Fox, a criminologist at Northeastern University. "But over the decades, there has definitely been an increase in this type of crime."

Fox and Northeastern sociologist Jack Levin, coauthors of "Mass Murder -- America's Growing Menace," analyzed 364 cases of mass murder from 1976 through 1985.

The researchers defined mass murder as killing four or more

victims within a short time.

The pair say there are, on average, three mass murders in this country each month.

They estimate 1,772 people died in these cases over the 10-year period, but the number pales when compared to the approximately 20,000 homicides in the United States each year.

The Federal Bureau of Investigation collects crime statistics on murderers, but it does not break out figures on multiple killings.

Fox said 1966 was "the onset of the age of mass murder." That was the year Richard Speck strangled and stabbed eight student nurses in a Chicago apartment and Charles Whitman killed his wife and mother, then climbed a campus tower at the University of Texas in Austin and shot 14 others to death.

Fox said that while the number of mass slayings fluctuates, several of the century's worst cases have occurred since 1980. Topping the list is the 1984 massacre of 20 people at a San Ysidro, Calif., McDonald's restaurant and the recent Christmas holiday deaths of 16 people in Russellville, Ark.

In 75 percent of the cases studied, the victims knew their killer, almost always a white male who often planned the killings for weeks.

Fox and Levin cite four other common threads running through most mass slayings:The killer was familiar with firearms. There usually was some precipitating event, such as loss of a job or divorce or separation from a spouse. The killer led a life of frustration filled with menial jobs and real or imagined slights. The killer had few outside contacts with friends or neighbors who might have helped vent the growing rage.

Levin notes that societal trends are creating more opportunity for these conditions.

"You look at the divorce rate of 50 percent and the tremendous residential and job mobility in this country," Levin said. "It can leave people rootless, isolated without sense of community."

Dr. Park Dietz, a psychiatrist at the University of Virginia and a consultant to the FBI's National Center for the Analysis of Violent Crime, has examined several mass killers and believes many suffer from depression and see no way out.

"I never came across one who wasn't at least partially interested in suicide," he said. "They have a very limited view of the options, such as a career change, divorce or declaring bankruptcy. The person doesn't feel they have the energy to pursue any of those options."