Authorities say Syed Rizwan Farook, 28, and his wife, Tashfeen Malik, 27 — the shooters who killed 14 people and injured 21 others this week in San Bernardino — had two legally purchased .223 caliber assault-style rifles and two 9 mm semi-automatic handguns. Police want to speak to a friend of Farook who purchased the two semi-automatic rifles used in the massacre, authorities told NBC News Friday.

For gun control advocates, the shooting, which officials announced Friday is being investigated as an act of terrorism, resurfaced long-time concerns.

“The way our laws are structured unfortunately often times allow people to legally buy guns who shouldn’t,” said Mike McLively, a staff attorney at the Law Center to Prevent Gun Violence based in San Francisco.

“The way our laws are structured unfortunately often times allow people to legally buy guns who shouldn’t.” Mike McLively, staff attorney at the Law Center to Prevent Gun Violence

Such was the case in other horrific mass shootings, including Virginia Tech in 2007 and the Charleston, South Carolina, church shooting in June: in both instances the records of the gunmen likely would have raised red flags had they been fully examined through the federal background check system.

A large part of the problem: When an individual goes to buy a firearm, they are subject to investigation under the National Instant Criminal Background Check System (NICS). A 72-hour hold is put on the sale.

But because federal law operates under the idea of “default proceed,” the sale can legally go through after 72 hours — even if a background check hasn’t been completed. About 9% of background checks require further investigation, according to the Law Center to Prevent Gun Violence.

Officials have not said whether there was anything in the San Bernardino shooters’ backgrounds that would have popped up in a background check when they bought their weapons, and little is known about the friend who purchased the assault rifles that they used.

Alan Lizotte, a professor of criminal justice at the State University of New York at Albany, said secondary firearm transfers — private sales of weapons between individuals that don’t involve a gun dealer or manufacturer — are difficult to track.

“By and large, those secondary transfers aren’t regulated at all,” he said.

And even when they are, states don’t always get the information they need. In New York, for example, the SAFE (Secure Ammunition and Firearms Enforcement) Act of 2013 requires a universal background check, meaning anyone who owns an assault rifle must register it with their county sheriff, whether it’s from a private sale or a gun dealer.