A married couple walked into an office party at a San Bernardino, California conference center on Wednesday and killed 14 people with guns that the San Bernardino Police Chief said were bought legally.

It's unclear who bought those weapons, how they wound up in the suspects' hands, and in what state they were purchased. What's clear is that California has some of the toughest gun control laws in the United States, including background check laws many progressive Americans want passed at a federal level.

Licensed firearms dealers throughout the country issue background checks for people looking to buy a gun, but many states don't require background checks when one private owner either sells or gives their gun to another person.

A man carries an AR-15 rifle at a gun rights rally in Pennsylvania, in 2013. The suspects in the San Bernardino mass-shooting reportedly used AR-15 rifles. Image: PennLive.com, Joe Hermitt/Associated Press

But California is different. Private owners can only sell a gun to another person through a licensed gun store, and buyers must file all the same paperwork.

The potential owner must be a California resident with some type of identification given to them by the California Department of Motor Vehicles, according to the state's website. Anyone 18 years old and up can buy a long gun, such as rifles and shotguns. Anyone at least 21 years-of-age can buy a handgun.

If someone is handing a gun over to an "immediate family member," however, they don't have to go through a licensed gun store.

Authorities investigate the scene following a shootout in San Bernardino, California, on Dec. 2. Image: Jae C. Hong/Associated Press

Potential buyers go through a 10-day waiting period while the dealer runs a background check.

California background checks should prevent anyone from purchasing a gun who has committed a felony. It also prohibits gun purchases by anyone found mentally unfit to be put on trial, and by sex offenders with a mental disability, according to state law.

The state also issues long-term prohibitions on gun ownership to people who have committed various misdemeanors or who are deemed unsafe to themselves or others.

In addition, California can issue undefined prohibitions to people addicted to drugs, charged with a felony, or who have voluntarily admitted themselves to a mental health facility.

In the wake of mass-shootings, politicians often call for tighter gun control legislation, and this time looks to be no different.

We're going to force the Senate to vote today on amendments that do something to stop gun violence. — Senator Harry Reid (@SenatorReid) December 3, 2015

Nevada Sen. Harry Reid has championed a bill that would prevent people on the United States' "no-fly" list from legally purchasing a gun. Right now, there are no such restrictions.

Dear Republicans, let's close the NRA loophole that lets terror suspects buy Paris-style assault rifles in America. https://t.co/GNhPde3D6Z — Senator Harry Reid (@SenatorReid) November 20, 2015

Those on the no-fly list are often monitored for relationships to terrorist networks, but may not have committed a crime.

Politicians such as Reid and President Barack Obama say this "loophole" could allow terrorists to legally purchase guns in the U.S. Others worry that preventing people on the list from buying guns would violate their constitutional rights.