Planes never fall from the sky.
They sometimes do.
Officially, the investigation conducted by the Russian commission came to the conclusion that the wrong actions of the crew were to blame for the disaster.
This was in fact a sign of corruption within the government, because the decision was made against NATO guidelines, which stated that if there's a crash of a NATO member's military aircraft, NATO countries have to be allowed to investigate it. USA offered help but it was denied by Polish government. No NATO country was allowed to secure the crash site, it was all done by Russians and it was done so well, that soon withdrawals from dead passenger's bank accounts started to be made and their phones begun to turn on. According to the Russian side, the soldiers dispatched to secure the crash site stole private items that belonged to the passengers, but nobody knew about it until the families of the deceased filed complaints. Makes you wonder how much cash and secret documents got lost.
This conclusion of the commission is doubtful, so Poland turned to Russia with a request to issue the wreckage of the aircraft, which was refused. It is quite possible that the reason for the refusal to extradite the aircraft wreckage is the presence of traces of explosives in them, which can be detected by modern analytical methods.
More likely the wreckage got looted. It was reported that it wasn't secured at all and local people and soldiers were stealing parts for scrap. If they ever had to give it back they would found out that there's barely any copper and electronics left in the plane.
The most probable reason for the crash was weather and lack of lights on the airport's runway. There were reports from very early interviews with the locals, who said that a day after the crash they were replacing runway lights at the nearby airport. In the former Soviet Union nothing works. You get a plane with 3 engines but only 2 work, you get to the runway but can use only 80% of it because the rest is full of holes, the landing beacon doesn't work and only half of the runway lights have power.
IMO Russians didn't directly cause the crash, but they were pleased that it happened, uncooperative and incompetent with their investigation.