The State Department said Thursday that it cannot explain publicly what changed that allowed U.S. officials to remove Iran’s national air carrier from a list of sanctioned entities, a move that could let Iran Air buy up to 100 Boeing aircraft in a deal reportedly worth $25 billion.

Five years ago, the Obama administration’s Treasury Department sanctioned Iran Air after determining that the airline is “used by the IRGC and Iran’s Ministry of Defense and Armed Forces Logistics (MODAFL), to transport military related equipment.”