According to this screenshot it was the Armed Forces of Ukraine that shelled the bus with civilians near Volnovakha, having informed about 10 dead and 3 captured terrorists, and later allegedly decided to accuse militants of so-called “Donetsk People’s Republic”.
However, there wasn’t such news that day. Firstly, there is no such post on the wall of this community. It can be proved by the cache of this page in Google.
In fact the screenshot of this page on the wall of the community “Ukrop | The Right Sector | News of the ATO” looks like that: