Thomas Brandon, who has led the Bureau of Alcohol, Tobacco, Firearms and Explosives since 2015, will retire, the bureau confirmed on Wednesday. Brandon will step down on April 30.

Deputy Director Brandon has faithfully served ATF for 30 years, starting as a Special Agent in 1989,” ATF spokesperson Bradley Engelbert said in an emailed statement. “He has earned his retirement.”

Brandon became the most senior official in the bureau on April 1, 2015. He has led in an acting capacity, since President Barack Obama never officially nominated him to be director, which would have required Senate confirmation. He has been the longest serving head of the ATF since Bradley Buckles, who was appointed in 1999 and served through January 2004.

Engelbert said the bureau did not yet know who would replace Brandon. Last November, Politico reported that Chuck Canterbury, the head of the Fraternal Order of Police, was being considered for the position. Canterbury did not immediately respond to an email seeking comment.

During Brandon’s time as director, the ATF was tasked with investigating some of the worst mass shootings in American history, including those in Charleston, San Bernardino, Orlando, Las Vegas, Sutherland Springs, Parkland, and Pittsburgh,

Brandon managed the ATF’s transition from the Obama administration, which sought to more strictly enforce gun industry regulations, to the Trump administration, which has been far more deferential to firearms makers. Despite rumblings by other ATF leaders at the beginning of the Trump administration that the bureau was considering reforms friendly to the gun business, including deregulating silencers, Brandon has largely refrained from changing its approach.

The deputy director leaves the bureau at a time when it faces a likely thinning of its ranks. As Brandon told the House Commerce, Justice, and Science Appropriations Subcommittee last month, the Trump administration’s budget for the ATF would likely result in the agency losing 377 staffers to retirement without any money to replace them.