Reason for explosion in Belgorod is now known: piece of ammunition fell from Russian bomber
Українська правда
The cause of the recent explosion in the Russian city of Belgorod was the fall of a piece of aviation ammunition during the flight of a Su-34 fighter-bomber of the Russian Aerospace Forces.
Source: Russian propaganda publication TASS referring to the Ministry of Defence of the Russian Federation
Details: According to the Russian ministry, residential buildings were damaged in the incident, but there were no casualties.
https://www.pravda.com.ua/eng/news/2023/04/21/7398793/
Source: pravda.com.ua