The reality, though, at least in the United States, is that mass killings rarely involve multiple shooters.
Of the 28 deadliest shootings in U.S. history before Wednesday
-- from Howard Unruh's 1949 rampage in Camden, New Jersey, to Christopher Sean Harper-Mercer's killing spree at Umpqua Community College in Roseburg, Oregon, in October -- only two have come at the hands of multiple shooters: the February 1983 killings at the Wah Mee gambling and social club in Seattle and the Columbine High School massacre in Littleton, Colorado, in 1999.
In Seattle, Kwan Fai Mak, Benjamin Ng and Wai-Chiu "Tony" Ng robbed 14 patrons of the gambling parlor before shooting each of them in the head. Thirteen of the victims died; the other survived to testify against the shooters. In Columbine, teenagers Eric Harris and Dylan Klebold killed a dozen fellow students and one teacher before committing suicide in the school library.
Such historical data reinforces the highly unusual nature of the massacre in San Bernardino, where suspects armed with AK-47-type weapons burst into a party hosted by the county health department and fatally shot 14 people, wounding at least 17 others.
Police identified the shooters as Syed Rizwan Farook, 28, and his wife, Tashfeen Malik, 27, who both were killed later in a shootout with officers. As of Thursday morning authorities had not offered a possible motive for the shootings.
The FBI has found
that of 160 "active shooter" incidents in the United States between 2000 and 2013 -- defined as a situation where law enforcement is responding to a shooting in progress -- all but two involved a single shooter.
On August 27, 2011, two men opened fire at a house party in Queens, New York, wounding 11 people, after one of them had gotten into an argument. And on April 6, 2012, two men fired from their vehicle as they drove through the streets of Tulsa, Oklahoma, killing three people and wounding two others.
It's also unusual that one of the San Bernardino suspects was a woman. Of the 160 active-shooter incidents recorded by the FBI, only six of the shooters were female, the report says.
While mass shootings by lone gunmen are often premeditated, killings by multiple shooters can suggest a higher level of planning, law enforcement officials say.
"They came prepared to do what they did, as if they were on a mission," San Bernardino Police Chief Jarrod Burguan said at a news conference Wednesday.