It seems clear that the plane crash which killed President Lech Kaczynski in Smolensk was the fault of the Polish pilots, however Warsaw was not too pleased by the level of cooperation during the investigation. From the Wall Street Journal:
Polish investigators sent their comments to Russia on Dec. 19, renewing requests for information on procedures and the compliance of the provincial and primitive airport in Smolensk with Russian standards. The requests were refused. Ms. Anodina explained Wednesday that the Russian report was technical in nature, which is why her committee decided not to reflect the comments Poland sent.
But the Dec. 19 document shows that Ms. Anodina’s committee had stopped responding to most Polish requests for information as early as May. This happens to be the time when the line from officials in Moscow and Warsaw was that the relationship between the two countries was getting warmer and that the investigation was going well.
Yet, on July 20, Poland said: “The Polish side hasn’t received answers to the questions asked on May 14 and later. When can we expect an answer?”
The answer never came.