Again we are visited by the horror and frustration of a mass shooting. We try to make sense of it, to see signs that someone should have seen before it happened, even to link the terrible event to the availability of the weapons used to carry it out.None of it works. We have to admit that something we don’t understand is happening, a social phenomenon that seems to be gathering steam and we don’t know how to stop it.Maybe all we can do for now is be aware that it happens and think about what to do if it happens in our own workplace or in a public place where we happen to be.This time, a 34-year-old man who until recently lived in Fort Worth allegedly sprayed bullets into a dining area at the Washington Navy Yard beginning shortly after 8 a.m. Monday, killing 12 and injuring others before being cut down in an exchange of gunfire with police.Aaron Alexis, a Navy reservist when he lived in Fort Worth, had been arrested in 2010 on suspicion of discharging a firearm, and he also had been named in a Seattle incident for allegedly having shot out two tires on a vehicle.Friends said he was frustrated over military pay issues. Reports Tuesday said he had been treated for mental illness.None of that teaches us very much. The FBI is searching for a motive.Should someone have seen warning signs in his behavior?Writer Richard J. McNally, in a 2011 article on Salon.com entitled “Why Psychiatrists Can’t Predict Mass Murders,” pointed out that even though these incidents seem to happen all too often, they’re still very rare.“Although mass murderers often do exhibit bizarre behavior,” McNally wrote, “most people who exhibit bizarre behavior do not commit mass murder.” Research hasn’t provided many answers. In March, the Congressional Research Service, a branch of the Library of Congress that provides policy and legal analysis for committees and members of both the House and Senate, issued a report on public mass shootings.CRS tailored its report to focus on incidents occurring in relatively public places, involving four or more deaths not including the shooter, victims selected somewhat indiscriminately. It did not count incidents in which the killings were a means to an end — criminal profit or terrorism, for example.The report identifies 78 public mass shootings in the United States since 1983. It tallied 547 deaths and an additional 476 injuries.Add one more incident and 12 more deaths in Monday’s Naval Yard killings.Public mass shootings happen more often at workplaces than at any other category of location. Places of education come in second.Shooters commonly act alone, are usually white males and are often killed during the shooting incident, by their own hand or by law enforcement officers. Their average age was 33.5 years.The gun control debate often flares after public mass shootings, but CRS points out that these shootings account for relatively few of the murders related to firearms in the United States.FBI data show 8,583 people were murdered with firearms in the U.S. during 2011. The Washington Post has reported that half of the 12 deadliest shooting incidents in U.S. history have happened since 2007.Whatever the outcome of the debate about who should be allowed to own guns and what guns they should be allowed to own, mass shootings seem to add little to informed opinion.Few things are known about what turns someone with a gun into a mass killer. Not much at all, in fact, that helps us know how to stop them before they act.