The Daily Mail article cited above shows a covered truck and gives a very vague story.
Even more importantly, it doesn't supply a motive.
What gain could Russia have in shooting down an airline?
There is no obvious motive (that I can see) and plenty of reasons not to. I've written an article here "Motive lacking in Russia blame for airline attack"in response to the Daily Mail allegations. I welcome ongoing discussion of the points I make at the base of that article, or here.
Korean airliner KAL 007 shot down by USSR in 1983
The Russian army is not unified and controlled as well as in other developed countries. Maybe the separatists bribed the army to get the weapons, or hijacked it. It needs investigation. Korean Air Lines flight 007 from New York to Seoul (after refueling in Alaska) flew into Russian airspace due to a navigational error and was shot down by Soviet fighter jets. All 269 passengers and crew, which included 63 Americans, were killed. There's an eerie parallel.
Editorial comment: There is much printed and on-line material to suggest that the KAL 007 incident was a staged provocation by the United States. One example is KAL 007 on what-when-how. It cites works on KAL007 including KAL 007-The Cover Up (1987), by David Pearson, which I own and much of which I have read. One outcome of the deaths of 269 people aboard KAL 007 was that public opposition to the deployment in Europe of Cruise and Pershing missiles directed against the USSR was overcome. - Ed