Agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives are being sent to retrieve more firearms from people who should have never been allowed to purchase them to begin with as growing gun sales have stretched the federal background check system, The Wall Street Journal reports.

Referrals for ATF agents to retrieve firearms wrongly sold to people have jumped since 2007, according to data from the Federal Bureau of Investigations, reaching 6,000-plus in 2017. The dilemma stems from a federal law that allows gun sales to proceed after three business days, even if a background check is not completed.

In 2017, 310,232 gun sales were allowed to proceed after a three-day limit compared to 303, 146 in 2016 and 271,359 in 2015.

Even more problematic, the ATF is one of the smallest federal law enforcement agencies with 2,600 agents.

Robert Cekada, special agent in charge of the ATF’s Baltimore office, told the Journal that each of his groups are short-staffed.

“When I have to pull one or two people or four people to turn around . . . a delayed denial, it complicates the situation,” Cekada told the Journal.

The report comes as Democratic presidential contenders step up calls for gun control legislation following a mass shooting in Illinois where a disgruntled employee fatally shot five people and wounded five officers in a warehouse. The shooter, Gary Martin, was a convicted felon and never should have been allowed to buy a gun.