Deadly July: Five mass killings in 10 days

Jodi Upton | USA TODAY

In the past 10 days, five mass killings have left 23 people dead in the U.S., making this one of the deadliest stretches of homicides that include at least four victims in several years.

Two people were killed in Thursday’s shooting at a packed theater in Lafayette, La., making it fall just below the level of a mass killing as defined by the FBI. But five other incidents since July 15, including shootings at two military centers in Chattanooga, Tenn., and three separate family killings — in Chicago; Modesto, Calif.; and Broken Arrow, Okla. — underscore how such incidents can cluster.

For example, in a 34-day period starting March 5, 2009, a string of 12 incidents across the country left 70 people dead. Only two of those were "public killings," which tend to get more coverage than incidents among families or during a robbery, making the string of violence less obvious. In that period, the higher-profile killings included eight people who were killed in a nursing home by a man who came in to shoot his wife, and 13 victims killed during a citizenship class in Binghamton, N.Y.

In another spate that started in September 2006, 52 people died in 10 incidents over a 36-day period.

Experts say mass killings can have a "contagion" effect lasting, on average, about 13 days. That can happen even when the killer is not directly aware of other incidents, as is often the case for the hundreds of mass killings that do not generate days of national coverage, says Sherry Towers, professor of statistics at Arizona State University’s Simon A. Levin Mathematical, Computational and Modeling Sciences Center.

But a string of incidents can also follow high-profile cases, such as the six that followed within two weeks of the April 2013 Boston Marathon bombing – nearly all of which were lower-profile robberies or occurred within families.

To hear more about the data, listen to NPR's Morning Edition.