The Phoenix Lights (sometimes called the "Lights over Phoenix") were a series of widely sighted unidentified flying objects observed in the skies over Arizona, Nevada in the United States, and Sonora, Mexico on Thursday, March 13, 1997.
Lights of varying descriptions were seen by thousands of people between 19:30 and 22:30 MST, in a space of about 300 miles (480 km), from the Nevada line, through Phoenix, to the edge of Tucson.
There were allegedly two distinct events involved in the incident: a triangular formation of lights seen to pass over the state, and a series of stationary lights seen in the Phoenix area. The United States Air Force identified the second group of lights as flares dropped by A-10 Warthog aircraft that were on training exercises at the Barry Goldwater Range in southwest Arizona.
Witnesses claim to have observed a huge carpenter's square-shaped UFO, containing five spherical lights or possibly light-emitting engines. Fife Symington, the governor at the time, was one witness to this incident; he later called the object "otherworldly."
The lights were reported to have reappeared in 2007 and 2008, but these events were quickly attributed to (respectively) military flares dropped by fighter aircraft at Luke Air Force Base and flares attached to helium balloons released by a civilian. (Source: Wikipedia)
As you can see from the above explanation, still touted by Wikipedia and major media outlets that have covered this story, the lights were blamed on flares dropped from aircraft, though as you can see below -- Luke AFB was clueless on such a theory -- and dismissed any flares were dropped.
Luke Air Force Base Official Response to The Black Vault, under the FOIA, while looking for records pretaining to the"aircraft flares" theory
Davis-Monthan Air Force Base Response regarding "aircraft flares" requested in August of 2013 [1 Page, 200kb] -- Whatever evidence, if any, has been destroyed.