Report of Incident — Granite City, Illinois, 13 February 1952

This document details an incident occurring on the night of 13 February 1952, involving an unidentified target tracked by Detachment 4 of the 3903d Radar Bomb Scoring Group at the Granite City Engineer Depot in Illinois. During a scheduled evaluation mission for the 97th Bombardment Wing, ground-to-air radar established contact with a target that initially appeared to behave like a normal aircraft. The target was locked in automatic tracking at approximately forty miles from the site. While the target initially maintained a course of 180 degrees, it later increased its speed and turned to a new heading of 197 degrees, beginning a descent. Radar data recorded the target's speed reaching up to 1090 miles per hour during its descent. The report notes that the target was eventually plotted at a horizontal range of about five miles west of the site.

Subsequent internal correspondence within the Air Technical Intelligence Center and the 3903d Radar Bomb Scoring Group indicates significant difficulty in evaluating the incident. While initial checks of the radar equipment at the time suggested it was operating normally, later assessments by the Radiation Section suggested that the radar data was insufficient for analysis. Officials speculated that the unusual returns might have been caused by radar malfunctions or atmospheric conditions, specifically temperature inversions and moisture lapses, which can cause excessive refraction and create false ground clutter signals. By May 1953, the Aerial Phenomena Section of Project Blue Book requested a review of the incident, but the final conclusion remained that the event could not be firmly evaluated. The investigators noted that it was highly doubtful that further investigation at that late date would yield useful information. The document includes various status reports and internal memoranda confirming the incident was treated as a matter of interest to the Air Technical Intelligence Center, though no definitive explanation was ever reached.