Twenty years ago, a pair of students killed a teacher and a dozen of their classmates at a high school in Littleton, Colo. The shooters at Columbine High School used semi-automatic weapons and sawed-off shotguns in the attack before turning the guns on themselves.
Just a few months before that shooting, the FBI launched the National Instant Criminal Background Check System to try to prevent dangerous individuals from purchasing guns.
And in the two decades since, the federal government says it has conducted hundreds of millions of background checks. With critical shortcomings in the system, though, mass shootings continue in the U.S.
"The weakness of the NICS system is talked about mostly in the wake of a tragic shooting, which happens more often than not," says Stephen Morris, a former FBI assistant director for the Criminal Justice Information Services Division, which oversees NICS.
NICS functions today much like it did 20 years ago. When someone wants to buy a firearm, a federally licensed gun dealer contacts the system. Usually within minutes, federal investigators receive the request and begin searching for clues within three main databases to approve or deny the purchase.
The FBI says the system has denied more than 1.3 million firearm transfers since NICS first began operating.
If more research needs to be done, the purchase can be delayed for up to three business days. If in that time investigators can't complete the additional background check, federal law allows the gun dealer to proceed with the transaction.
In 2015, a white supremacist in Charleston, S.C., was able to obtain a gun after the FBI failed to complete his background check before the three-day deadline.
The gun purchase was able to go through and that shooter later killed nine black churchgoers.
As NPR reported later, it was discovered the shooter had admitted possessing a controlled substance during an arrest. That should have been sufficient to deny the purchase on the grounds of "an unlawful drug user or addict."
Since it was created in late 1998, NICS has initiated some 311 million background checks, including 26 million in 2018 alone, the federal government says.
"We're talking about a system and a process that was created over 20 years ago. It goes without saying the system is stressed out," says Morris, the former FBI official.
The NICS system relies on state and local agencies to make the data more robust, but sometimes records can be missing or incomplete.
"Like any database, the system is only as good as the records put in that system," says Lawrence Keane of the National Shooting Sports Foundation. "There are some very real world examples of where the background check system was not accurate."
Consider the Virginia Tech shooting in 2007, in which 33 people died, including the gunman.
In that case, Virginia court documents released after the rampage showed a judge had previously declared the man "mentally ill" and ordered him to seek treatment. But at the time, Virginia wasn't fully sharing information with NICS. Had that information been in the system, there is a better chance the gunman would not have been able to obtain his guns.
In 2017, a gunman killed 27 people including himself in Sutherland Springs, Texas. He was an Air Force veteran and purchased an assault-style rifle and two handguns, but a domestic violence conviction during his time of service should have barred him from possessing the weapons. The Air Force never entered that information into NICS.
Following those attacks, Congress passed legislation to try to address the lapses, including offering states more financial incentives to share information with NICS.
After high-profile shootings, gun violence prevention advocates often renew their calls for universal background checks. Those would require background checks on virtually all gun sales, not just ones administered by federally licensed gun dealers.
Keane, with the gun industry trade association, says he doesn't think federal universal background checks are the solution.
"We think we have to work to fix the NICS system before you even have a conversation about expanding it," Keane says. "That does not make sense to us to expand background checks, through so-called universal background checks, when the system we have now is not working as it should be."
But 20 states, including California, Connecticut and Vermont, extend background checks to include private sales of at least some firearm transactions. That's according to Giffords Law Center to Prevent Gun Violence.
Just this week, an 18-year-old woman, who authorities say had an "infatuation" with the Columbine High School shooting, flew from Miami to Denver and within hours purchased a shotgun along with ammunition.
More than a dozen school districts in the Denver area, including Columbine High School, were closed Wednesday while a "massive manhunt" was underway for the woman. Her body was recovered later that day, after she apparently killed herself.
The gun shop owner who sold the guns posted on Facebook that the woman passed both the Colorado Bureau Investigation background check system as well as NICS.
"She did go through the full background check, and was given a clearance by both NICS and CBI," wrote Josh Rayburn. "We had no reason to suspect she was a threat to either herself or anyone else."