Russian news agency ITAR-TASS reports there were bars on the windows of the single-storey hospital building which is located about 70 miles outside Moscow. Some patients apparently died while trying to make it to the main entrance to escape. Many others died in their beds.
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The band announced at a free concert on Friday that they would take over the event's Pyramid stage on Saturday June 25.
The victims were collecting permits and escorts a day after their home town was liberated from the extremists.