After the September 11 attacks, there was an immediate call to action regarding the state of aviation security measures as the hijackers involved in 9/11 were able to successfully pass through security and take command of the plane. The existing security measures flagged more than half of the 19 hijackers in 9/11; however, they were cleared to board the plane because their bags were not found to contain any explosives. In the months and years following September 11, 2001, security at many airports worldwide were reformed to deter similar terrorist plots.