Tashfeen Malik, 27, and her husband Syed Rizwan Farook, 28, who had a 6-month old daughter together, were killed in a shootout with police after the massacre at the Inland Regional Center social services agency in the city of San Bernardino.

Research conducted by the Federal Bureau of Investigation, the New York Police Department and others show that mass shooters in recent decades are overwhelmingly male.

According to a report from the NYPD, only eight of 230 'active shooter' cases in the United States from 1966 to 2012 involved female attackers.

A study of 160 active shooter situations between 2000 and 2013 by the FBI showed that just six involved women.

Women shooters are remembered easily by experts, in large part due to their rarity. Brenda Spencer killed two and wounded eight children in a California schoolyard in 1979; Laurie Dann terrorized the village of Winnetka, Illinois, in 1988, tried to firebomb a school and shot six children, killing one.

There was so far no hard evidence of a direct connection between the shooters and any militant group abroad, but officials close to the investigation said electronics found at a townhouse used by the couple would be checked to see if they browsed militant websites.

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