What could cause an explosion in the air?
a surface-to-air missile was launched;
an explosive device that was planted on the aircraft (sabotage);
a tech malfunction (unlikely, because if something had gone wrong with the engine, there would have been some kind of smoke trail coming from it).
The distance from Kharkov (Ukraine) to Belgorod (Russia) is 70kms. And the Ukrainian rushed to claim Kiev deliberately shot down the Russian plane using the S-300 complex.
The Armed Forces of Ukraine have several other air defense systems that can shoot down an aircraft at a distance of 70kms:
Patriot,
Iris-T,
NASAMS.