Ronald Reagan Airport in Washington DC grounded all flights Tuesday after an alarming security threat forced an FAA emergency lockdown. The issue involved United Airlines Flight 512 arriving from Chicago. Police and emergency crews rushed to the runway. At least 12 flights were diverted to Dulles, Baltimore, Philadelphia, and Pittsburgh. The FAA confirmed an “unconfirmed threat.” Flights resumed after 5 p.m. ET, but delays topped 90 minutes. No injuries reported. Investigation continues as Ronald Reagan Airport remains under heightened alert.