The weapons used by the San Bernardino shooters may have been legally obtained, but that doesn't mean they remained legal under California law. The two .223-caliber semiautomatic weapons and two 9mm pistols said to have been carried by Syed Rizwan Farook and Tashfeen Malik in Wednesday's attack were originally purchased legally (the two handguns by Farook, the two rifles by a still-unnamed buyer), with all required federal background checks in place, Meredith Davis, an ATF special agent, tells the Wall Street Journal. But what happened to the two AR-15 rifle versions after their purchase is what put them on the wrong side of the law: Though the weapons were sold as what the Trace calls "California compliant" models with magazine-locking devices, Davis notes that the Smith & Wesson rifle used in the attack was modified in an effort to allow for fully automatic mode, while the DPMS rifle was altered so it could sport a larger-capacity magazine.
The state's laws allow for ample wiggle room, with the Trace noting that, thanks to the AR-15's "modular, customizable construction," a "determined owner" of such a weapon "would have other options." A Long Beach lawyer points out to the New York Times that a "substitution effect" makes it easy for people to legally buy stripped-down versions of otherwise illegal weapons. "If you ban Rifle X, people will use Rifle Y," he notes. In the meantime, authorities are trying to hunt down who originally bought the rifles, purchased in 2011 and 2012, per law enforcement specialists who filled in congressional officials on Thursday. The Times notes the shooters could have then obtained the rifles from the original buyer without going through the requisite background checks. (The Trace offers a more detailed explanation on how legal guns can be altered to skirt the law.)