When the man who would go on to commit the Odessa, Texas, mass shooting went to a gun dealer and tried to buy a firearm, the system worked as designed. An itinerant oil worker, he had a history of volatile behavior, and the records reviewed by the FBI examiner handling his gun background check indicated he was barred from gun ownership because of a serious mental health issue. The sale was blocked, and he went home empty handed.

But the state of Texas doesn’t require background checks for private sales. And according to a local ABC station, that’s how the Odessa gunman acquired an AR-15-style rifle. On August 31, he used it to shoot 32 people in a multicounty spree that left seven people dead.

Texas isn’t alone — 29 states don’t require any screenings on gun sales between individuals, according to Giffords. That means that sales facilitated at gun shows or arranged online may happen without government oversight, as long as they occur between private parties.

When someone wants to purchase a gun from a licensed gun dealer, they must submit to a screening by the FBI’s National Instant Criminal Background Check System, or NICS. The federal government blocks gun sales for 12 reasons. (Some states choose to run their own checks.) The FBI says its background checks have a 99 percent accuracy rate. Since 1998, NICS has blocked more than two million transactions.

Those safeguards are not in place for unregulated private sales. The gap in the law has created a thriving gray market for firearms and provides a means for people with criminal records or other disqualifying factors to easily access weapons. According to research by the Harvard Injury Control Center and Northeastern University, roughly one in five American gun owners obtained a gun without undergoing a background check.

Scot Thomasson, a former agent with the Bureau of Alcohol, Tobacco, Firearms and Explosives, who worked trafficking cases for decades, said private sales make it easy for people with histories of crime or violent behavior to avoid witnesses and make prosecutions difficult. “They work by creating anonymity for the purchaser,” he said. “Drug dealer, rapist, murderer — private sales are a means for criminals to skirt legal means to obtain [firearms].”

Universal background checks enjoy broad public support, but have faced stiff resistance from Republican lawmakers in Washington, D.C., and in state capitols across the nation. As the frequency of mass shootings has intensified, there are suggestions that conservative lawmakers may be more open to considering the policy as a means to reduce gun violence.

Last month, on the heels of consecutive mass shootings in El Paso, Texas, and Dayton, Ohio, President Donald Trump spoke favorably of universal background checks, calling them “important.” Trump later walked back his comments after speaking with the National Rifle Association.

Senate Democrats have called on Senate Majority Leader Mitch McConnell to support an expansion of background checks for private sales, as well. McConnell said he would discuss possible legislation, but said he would only bring it up for a vote if the president says he’d sign it.

Even Republicans in staunch Second Amendment-supporting states like Texas have begun to waver on their stance. Governor Greg Abbott broke the news that the Odessa gunman failed a check on Twitter:

Before the news of private sale to the Odessa gunman was confirmed by journalists, Lieutenant Governor Dan Patrick told The Texas Tribune that “we don’t know how this person got the gun — but we do know that that’s a real loophole in the law, and I think the NRA needs to get behind the president on that issue and really address that issue.”