One man whose daughter survived the explosion told Moscow Echo radio: "It was a powerful explosion - a huge blast. There were lots of students on the bus."
Another man, who was driving behind the bus, told Rossiya-24 television: "There was a blast - a bang - all the glass flew out of the windows. The cloud of smoke quickly dissipated and then I saw people start to fall out and run out to escape the bus. It was a horrible sight."