Lightning can sometimes produce short but powerful bursts of high-energy radiation called terrestrial gamma-ray flashes (TGFs). These events happen in the atmosphere and usually last less than a thousandth of a second. TGFs have been seen during winter thunderstorms in Japan, but until now, scientists have not been able to directly measure where the gamma rays come from. To study this, we set up new detectors in Kanazawa, Ishikawa, Japan, starting in the winter of 2022. These detectors can sense the direction of light produced when gamma rays hit small acrylic rods, helping us estimate where the rays came from. Each detector consists of four rods with sensors that help pinpoint the source with a few degrees of accuracy. On December 8, 2024, we detected a strong TGF lasting about 20 microseconds during a thunderstorm. It was picked up by four detectors placed in different areas of the city. Using radio signals from the lightning and our detectors, we found that the gamma rays likely came from an altitude of 1–2 kilometers. This is one of the first times we have been able to estimate the actual location of a downward TGF in such detail.