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.
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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.
"It surprised me; it has not been the pattern," said Brian Jenkins, an expert on militant attacks and senior adviser to the president of Rand Corp. "It is very unusual. Shootings tend to be a male activity."
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.
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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 terrorised the village of Winnetka, Illinois, in 1988, tried to firebomb a school and shot six children, killing one.
The reasons why women are rarely shooters are myriad, experts say, and it is hard to pinpoint any single factor. Some say men are more likely to express indiscriminate rage outwardly than women or that they identify more with guns.
"Our gun culture is very heavily masculine," said Deniese Kennedy-Kollar, assistant professor of criminal justice at Molloy College in Rockville Centre, New York. "It is seen as manly to be identified with guns."
Much remains unclear about Wednesday's attack, including Malik's exact role and motives. The couple had more than 6,000 rounds of ammunition and a dozen pipe bombs on them and elsewhere, authorities said on Thursday as they sought to determine if the pair had links to Islamic militant groups.
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.
CNN, citing law enforcement sources, said Farook had been "radicalised" and had been in touch through telephone and social media with more than one international terrorism suspect who was being investigated by the FBI.
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A couple acting together is very uncommon, experts said. Some wondered if Malik had gone along with a plan hatched by her husband. It was his work party that was targeted.
"In a case like this, almost always the male is the driving force," Kennedy-Kollar said.
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