Katie Murray, 13, died in South Warwickshire Hospital, Warwick, with her parents by her side. Her death makes Thursday's accident the worst on a British motorway involving the passengers of one vehicle.
After a day examining debris and the crash scene, police were no nearer finding a cause. Peter Joslin, Chief Constable of Warwickshire, said evidence ruled out driver fatigue, speed, weather and the condition of the converted Ford Transit van. 'We may never be absolutely sure what happened,' he said.
Ten children and a teacher, Eleanor Fry, 40, from Hagley Roman Catholic High School near Stourbridge, Hereford and Worcester, died just after midnight when the minibus crashed into a stationary motorway maintenance vehicle. Another pupil pulled alive from the wreckage, Charlene O'Dowd, 12, died on Thursday night.
The last survivors, Holly Caldwell and Bethan O'Doherty, were last night comfortable in South Warwickshire Hospital.
Mr Joslin said two witnesses had seen the minibus in the outside lane before the accident. 'What the investigators have to resolve is why did that vehicle move from the third lane on to the hard shoulder?' he said. 'There were no obstructions, visibility was good and there were no other vehicles on the motorway there at the time.'
Mr Joslin said Miss Fry, who had driven tens of thousands of miles across Britain and in Europe on school trips, had been driving for only two hours and had eaten before setting off. A post-mortem examination had found nothing that could have contributed to the accident.