According to Kommersant (a rather respected newspaper), the investigators have established that pilots (for so far unkown reasons) weren't able to use ILS on approach.
Link (Russian)
(Edit) Some more details from the report:
- Pilots didn't inform ATC that ILS wasn't used
- Apparently, there are indications on CVR that a GPS navigation system was used instead
- Last message from ATC to pilots was that they were left of glideslope, crash occured while they tried a course correction