Recent incidents differ, but experts say there could be a trend.
Details continue to emerge from Binghamton, New York, where a gunman identified as Jiverly Voong, 41, barricaded the back door of the American Civic Association Friday morning, then went in the front door shooting at everyone in the room, killing 13 and then shooting himself.
Early reports say the gunman was deeply upset over being laid off and for being disrespected for not speaking English well.
That event, as well as three policemen wounded in a Pittsburgh shooting after responding to a domestic disturbance call – friends said that gunman was also upset about his recent firing – fit a larger pattern of mass killings which have seemed to proliferate since America's economic downturn, experts say. Forty-four people have died in a string of five such incidents in the past month, from Oakland, California to Alabama to North Carolina.