Of all the shocking details emerging after Wednesday's mass shooting in California, one stands out as highly unusual: One of the two attackers who shot dead 14 and wounded 21 others at a holiday party was a woman.

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.