In mass killings, more than one shooter is extremely rare

Of the 28 deadliest shootings in U.S. history before San Bernardino shooting - 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.

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.

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You forgot about Mitchell Johnson and Andrew Golden