The two AR-15 rifles used by Syed Farook and Tashfeen Malik in Tuesday’s San Bernardino mass shooting were illegally modified, the ATF determined. The Trace reported yesterday afternoon that police descriptions of the scene suggested the shooting was carried out with weapons that skirted California firearms law. Then last night, The Wall Street Journal confirmed this hunch: one of the rifles had been altered to quickly release and accept magazines, which is illegal under California law. Investigators also found evidence that one of the rifles had been changed with the intent of making it fully automatic, a practice banned under federal law.

The two suspects in the San Bernardino shooting “sprayed the room” with more than 70 rounds from their AR-15 rifles in the course of their rampage, San Bernardino’s Chief of Police Jared Burguan said in a press conference Thursday. In addition, law enforcement officials found four “high capacity” magazines left at the scene when first responders arrived four minutes after the shooting began. In a final disclosure about the attackers’ weapons, ATF sources said the guns were purchased legally, four years ago, from stores in Southern California.

The suspects used two different brands of AR-15, one made by Smith & Wesson, and the other by a smaller manufacturer called DPMS, both of which were reportedly purchased by a third party. The Smith & Wesson rifle was modified in an attempt to make it fully automatic, and the DPMS rifle had been changed to quickly accept and release magazines. California bans the most common versions of the AR-15, which come with detachable magazines that allow for quick reloading with the touch of a button. To get around that law, many companies — including both Smith & Wesson and DPMS — make “California compliant” models with fixed magazines that can’t be released from the body of the gun with the press of a button. Often with these California models, shooters must open up the gun’s receiver and reload rounds one by one.

There are some partial workarounds, of course. So-called “bullet button” models of the AR-15 have a recessed magazine release button that can’t be pressed with a finger, but rather requires a small object, like the end of a bullet or some other tool. The feature is legal in California and faster than opening up the entire gun, but bullet button models are still slower and clumsier than a standard AR-15 available just over the state line in Arizona or Nevada.

A determined owner of a California-compliant AR-15 would have other options, however. The AR-15 is beloved for its modular, customizable construction, and the versions legal in the state are no different. One of the ways the guns can be tailored to suit the user is to outfit them with a standard magazine release button. Some companies make magnetic tools that can sit atop a recessed bullet button to enable magazine release with just a fingertip. One such company stresses that their products should not be used within California but only for “out of state…’Emergency Zombie Attacks,’” since it’s not clear whether they are legal in California. With these simple changes or parts affixed, a shooter can easily insert and release the 30-round magazines favored by AR-15 owners elsewhere.

It’s not yet known whether Farook and Malik used commercially available products like these, or took a more DIY approach in modifying their weapons. Parts that change guns from semiautomatic to fully automatic are considered machine guns in their own right, and have been strictly regulated and hard to come by since the passage of a 1986 federal law. These simple parts do require some technical knowledge to successfully install.

Burguan, San Bernardino’s police chief, did not specify how large the magazines found on the scene were. But a spokesperson for the FBI’s Los Angeles field office says that a typical large AR-15 magazine holds 30 rounds. That’s 20 more than allowed under a state law enacted in 2000, though possession of older magazines that exceeded the limit is still okay. If California Lieutenant Governor Gavin Newsom has his way, even those grandfathered magazines would be outlawed. He’s currently gathering signatures for a 2016 ballot initiative that would completely bar California gun owners from possessing magazines that hold more than 10 rounds of ammo.