'Until further notice': Why has FAA grounded all flights across US and what's behind the nationwide shutdown?

US flight outage: The FAA announced ⁠the nationwide ground stop at 3:54 p.m ET (2054 GMT). At the time the ground stop was issued, 4,042 flights were airborne, destined for US airports. This is the second such shutdown since the September 11, 2001, attacks.