Ann Jasper, 22, broke down in tears as she spoke publicly for the first time about events in the youth hostel at Pleine Fougères in Brittany in July 1996.
"I could hear Caroline mumbling 'Mum' and 'Help' or something like that," she said yesterday. "I could see her wriggling in bed but I thought she was dreaming and I went back to sleep."
Earlier Ms Jasper said that she had been woken up by the sound of "something banging". "I thought it was a bottle top hitting the wall," she said.
Francisco Arce Montes, 55, a Spanish drifter, is appealing against a 30-year jail sentence and his conviction last year for Caroline's murder. He admits raping the girl and accepts responsibility for her death but denies that he meant to kill her.
The assize court in St Brieuc, Brittany, has seen a quite different Montes from the sullen and taciturn man who appeared at the first trial in Rennes last June. In the first three days, Montes has been almost unhelpfully talkative, butting into the proceedings to explain points of fact.
His new-found spirit of co-operation faltered, however, when he was asked to look at pictures of the dead girl's body. "Photos of Caroline? It's not possible. It's horrible," he said.
"You are playing the outraged virgin? You?" said the president of the panel of judges, Jean-Luc Bockel.
The case is expected to end on Tuesday. If Montes's murder conviction is reduced to manslaughter, he may be given a shorter sentence. But by bringing the appeal he risks having his sentence increased.