On 8 August 2014 these four investigating nations signed an agreement that
the results of the investigation would not be published unless all four countries agreed. This gave one of the prime suspects in the atrocity, Ukraine, an effective veto over any investigations result that attributed blame to them. This is an astonishing situation and probably without precedent in modern air crash investigations.
Why the Secrecy on the Mh17 Investigationhttp://www.counterpunch.org/2014/12/19/why-the-secrecy-on-the-mh17-investigation/From the article:
The Russians further disclosed that at the precise time of the shoot down an American spy satellite was directly overhead the scene and would have recorded the sequence of events. The Russians invited the Americans to share these data with the official investigation that had been launched
It seems that back then the Americans were shocked by this fact (and more than likely still are), and had been panicking (and are still afraid) that the next evidence the evil Russians could reveal would be the snapshots right from this reconnaissance satellite.