Just prior to the September 11 attacks – between November 2000 and April 2001 – French aviation authorities noted and investigated repeated instances of unexplained commercial airliner Mode S transponder jamming resulting in the loss of secondary radar data to Air Traffic Control (ATC). Affected aircraft included Air France Boeing 767s.
Three of the four reportedly hijacked September 11 flights also stopped providing secondary radar data to ATC via Mode S transponders during their flights. These Mode S transponder data losses were officially attributed to manual deactivations by alleged hijackers, although evidence to confirm this claim was never cited by the 9/11 Commission Report. September 11 flight transponder data losses contributed significantly to a circumstantial impression of alleged hijacker flight deck takeovers and also coincided with significant nationwide US military exercises that day.