It's military, so no public acess to investigation papers.
Except this short notice:
https://ria.ru/inquest/20110513/374115628.html
And this pathetic article (summarized rumors, leaks and some forum discussions):
Tu-154
Translate with Google.
In short, there was some wrong wires connection at EFCS which cause inversion of pitch and roll compensation signals.
PS: A bit reminds this one:
https://en.wikipedia.org/wiki/China_...es_Flight_2303 and this:
https://ru.wikipedia.org/wiki/%D0%9A...86%D0%BA%D0%B5