The rifles used in the San Bernardino mass shooting were illegal under California law because they were modified and violated the state’s ban on assault weapons, the Bureau of Alcohol, Tobacco, Firearms and Explosives determined on Thursday.

Syed Rizwan Farook and Tashfeen Malik, the suspects in the Wednesday shooting that left 14 people dead and 21 wounded, were armed with four guns. They carried two .223-caliber semiautomatic weapons and two 9mm semiautomatic pistols, according to San Bernardino Police Chief Jarrod Burguan.

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