The FBI issued more than 4,000 orders last year to reclaim guns from buyers who should have been blocked by the federal background check system from buying firearms.

Citing FBI records, USA Today noted in a Monday report that a total of 4,170 orders were issued by the FBI in 2016 to agents from the Bureau of Alcohol, Tobacco, Firearms and Explosives to seize guns from buyers with criminal records, mental health issues and other red flags that should have disqualified them from purchasing firearms.

This was the largest such seizure request in a decade, the report noted, adding that the actual number of banned guns could have been higher since more than one firearm can be purchased in a single transaction.

It remains unclear how many guns were successfully retrieved.

Investigation: 1000's of guns were sold to people without @FBI background checks. Now, @ATFHQ must get them back. https://t.co/N5fgq2GDK1 pic.twitter.com/QNGh0YcEWI — USA TODAY Video (@usatodayvideo) December 4, 2017

The revelation comes just days after U.S. Attorney General Jeff Sessions ordered a federal review of the FBI’s National Instant Criminal Background Check System, the database used to conduct background checks on gun buyers.

Sessions called for the review after it emerged that Devin Patrick Kelley, the former Air Force serviceman who shot and killed 26 people at a Texas church last month, had been excluded from the database despite a criminal record.

The Air Force admitted it had failed to enter Kelley’s criminal history into the database.

The FBI reviews millions of gun transactions each year ― and sometimes even hundreds of thousands of transactions per day. Under federal law, however, the FBI has just 72 hours to conduct a background check ― after which time, the gun seller is allowed to complete the sale even if the check has not concluded. The bureau can later seize the guns purchased if it turns out the sale should have been barred.

The FBI has said that this so-called “default to proceed” loophole allowed convicted mass murderer Dylann Roof to buy one of the weapons used in his deadly 2015 rampage of a South Carolina church.