[hindsightbias]

Most civilian ATC radars are 'Secondary' radars which depend on cooperative transponders on all aircraft. If you turn your transponder off, the system doesn't work.

US ATC uses 'Primary' radars as backup for locating aircraft that don't have transponders or don't have them on. Militaries use Primary radars for most of their activities. Other articles don't provide distinction on what type of radar was used.