U.S. and Canadian accident investigators are uncertain how much access they will get to the site of a Ukrainian passenger jet crash in Iran, and there were fears Friday that the probe might already be compromised by the removal of wreckage.

The U.S. and its allies alleged earlier this week that the American-built Boeing 737 was brought down by an Iranian anti-aircraft missile.

Iran announced Saturday that its military “unintentionally" shot down the jetliner, blaming “human error.”

All 176 people aboard were killed when the plane went down in flames on the outskirts of Tehran during takeoff early Wednesday, hours after Iran launched a barrage of missiles against U.S. forces.