Under Brandon's leadership, the ATF investigated some of the nation's deadliest mass shootings. The ATF also expanded a national network that photographs bullet casings to match that information against evidence collected at other crime scenes.
And the ATF has begun enforcing the Trump administration's ban on bump stocks, devices that allow semi-automatic weapons to fire like machine guns. Brandon's retirement comes weeks after he told Congress he was concerned a budget proposal would further strain the agency's resources.
There's no immediate word on who will replace Brandon.