Gun control advocates have pressed the White House to expand the federal government’s interpretation of who is “in the business” of selling guns, and Democratic presidential candidate Hillary Clinton has vowed to take executive action on the matter should she reach the Oval Office.

But navigating the process for closing the loophole unilaterally has proved difficult for the White House.

“Taking Administrative action in this space is enormously complicated — with complex (and intertwined) policy, legal, and operational considerations to take into account,” one official said. “But that process is very much underway.”

It’s not clear that closing the loophole would have prevented the San Bernardino shooter from obtaining the weapons used in this week’s massacre, which he obtained legally. The White House insists action to tighten current laws is still worthwhile.