The 1952 Washington, D.C. UFO incident, a.k.a. the "Invasion of Washington", was a series of UFO reports from the 12th to the 29th July 1952 over Washington DC.
At 11:40 p.m. on Saturday, July 19, 1952, Edward Nugent, an air traffic controller at Washington National Airport (today Ronald Reagan Washington National Airport), spotted seven objects on his radar.
At 8:15 p.m. on Saturday, July 26, 1952, a pilot and stewardess on a National Airlines flight into Washington observed some strange lights above their plane. Within minutes, both radar centers at National Airport, and the radar at Andrews AFB, were tracking more unknown objects
The official statement, especially about the peak of the wave, was that the causes were temperature inversions and mirages.
At the request of the Air Force, the CAA's Technical Development and Evaluation Center did an analysis of the radar sightings. Their conclusion was that "a temperature inversion had been indicated in almost every instance when the unidentified radar targets or visual objects had been reported." Project Blue Book would eventually label the unknown Washington radar blips as false images caused by temperature inversion, and the visual sightings as misidentified meteors, stars, and city lights.
The history of this story was also covered in a 2011 article in The Guardian.
Seeing all the details and reports, radar data, etc., I feel very skeptical about such an explanation.