The couple behind the murderous attack in San Bernardino had amassed an arsenal of several thousand dollars worth of arms and ammunition in a state, California, that has some of the toughest gun laws in the US.
Syed Farook and his wife, Tashfeen Malik, used two semi-automatic assault rifles in the shooting Wednesday that left 14 of Farook's co-workers dead and 21 wounded, police said. Farook and Malik died in a shootout with police.
The rifles, which police said had been purchased legally, were both variants of the AR-15, one manufactured by Smith & Wesson and the other by DPMS Panther Arms; the shooters also used two semi-automatic handguns.
Investigators are continuing to trace the history of the weapons.
California has banned semi-automatic rifles (automatic weapons, which can fire a steady stream of bullets with one steady squeeze of the trigger, are banned for the general public in the US).
But gaps in the state's legislation have allowed arms makers to circumvent the law easily and sell weapons -- known in the gun trade as "California compliant" -- that are slightly modified for the state's market.
The most notorious gap in the law is the ban on detachable magazines. Manufacturers get around it by adding a mechanism, specially for California, known as the "bullet button." It allows the user to rapidly detach the magazine by using the tip of a bullet to press on a button.
Since the bullet is considered a "tool," the magazine is not classified as detachable, though in practice it allows the user to reload quickly.
California remains, nevertheless, a model for gun-control advocates. Indeed, Laura Cutilletta, a lawyer for the Law Center to Prevent Gun Violence, says California's gun laws are the most restrictive of the 50 states.
But some of California's neighboring states are among those with the laxest gun laws, underscoring the challenge facing gun-control advocates.
Arms purchases in California are subject to checks into the criminal and psychiatric backgrounds of potential buyers, while in many states sales over the Internet or at gun shows take place with no verification, largely unrestricted by federal law.
California bans the sale of magazines holding more than 10 rounds of ammunition. Handgun purchases are limited to one per person per month.
Each purchase is subject to a 10-day waiting period. And every gun owner must obtain a Firearm Safety Certificate after passing a written exam.
In 2007, California became the first US state to pass a law on what is known as the "micro-stamping" of semi-automatic pistols, a technique that etches marks into each cartridge so that it can be traced to the weapon's owner. The law took effect in 2013, but the pro-gun lobby has challenged it in court.
This year and last, Cutilletta said, the state legislature adopted no fewer than nine gun-control measures.
In September 2014, Governor Jerry Brown signed into law a bill authorizing a judge to order that a person's weapons be seized immediately if there is reason to believe the person poses a risk to himself or to others, particularly if the request comes from a relative -- a much broader standard than currently prevails.
That law was passed after a mass killing last year in Isla Vista, near Santa Barbara in southern California. Elliot Rodger, a college dropout who had long suffered from mental illness, killed six people. The law is set to take effect on January 1.
California shares borders with Nevada and Arizona, both with far more lenient gun laws. The Law Center to Prevent Gun Violence gives both states its worst grade: F.
