There should be missile fragments amongst the ruins. Their discovery would yield what missile it was and likely where and when it was manufactured. There is therefore a very strong incentive for the perpetrators to not to allow independent inspection of the wreckage.
If anyone was actually questioning who fired the missile that might be the case.
Why would the Ukrainians fire a SAM when the rebels don’t have any aircraft? Are the Russians going to state that they are flying air support for the rebels in order to cast doubt on who shot the airliner? I doubt it.