The Federal Aviation Administration is investigating after a United Airlines flight and a Delta Airlines flight had a near-collision over Phoenix on Saturday.

The Federal Aviation Administration (FAA) said it is investigating a recent near-midair collision of flights that were approaching Phoenix Sky Harbor International Airport. A Delta Air Lines flight, as well as a United Airlines flight “landed safely” following “a loss of required separation” while both en route to the Phoenix airport,

The United and Delta flights were less than a quarter of a mile apart horizontally before the warning devices went off, officials say.

The two flights, United Airlines Flight 1724 and Delta Air Lines Flight 1070 came too close to each other while arriving at the airport.