Indeed, female shooters in the U.S. are a statistical anomaly. A 2014 FBI report revealed that only six of the 160 “active shooter” incidents between 2000 and 2013 were perpetrated by women.

Other female mass shooters in the last 40 years can be counted on one hand: Amy Bishop, who shot six University of Alabama Huntsville colleagues at a faculty meeting in 2010, killing three of them; Jennifer San Marco, a one-time U.S. Postal Service worker who in 2006 murdered six employees at the processing plant where she used to work; Brenda Spencer, who was 16 when she fatally shot two adults and wounded eight children at a San Diego elementary school in 1979.

Where these mentally ill women had “snapped,” Malik demonstrated more skill and apparent deliberation in her attack with Farook.

If the mass shooting does turn out to be an act of terrorism, experts say it isn’t unusual for women to have prominent roles within extremist groups.

“There is a possibility that Malik pushed Farook from being a devout, moderate Muslim to a more radical one,” said Mia Bloom, professor of communication at Georgia State University, who specializes in the role of women in terror organizations.