Release date: 1980 - 2000.
The authorities cover it up, they say the boy died due to a fall, they won’t allow the parents to see the body.
Many years pass.
Someone is still investigating this and they find a tape recording of a phone-conversation that includes somebody reading out a timestamp (e.g.: two hours, twenty minutes, three seconds) and they find it suspicious that it includes the seconds, which turns out to be some secret code for the dumped toxic waste.
Casspir has already found this once in 2018. June 29th on namethatmovie.org but I don’t remember the answer and Wayback Machine did not archive it.
http://namethatmovie.org/36582/investigation-movie-about-secret-environmental-pollution