Claire Hart from Congleton, Cheshire, knew and trusted Craig Smith, the man accused of her murder. That, said the prosecution at Chester Crown Court, was "a terrible mistake".
Mr Smith had told his parents five days before the murder: "There's a rumour I've killed a girl." And when he fell into conversation with two young boys in woods near the scene of the killing, Mr Smith pulled out a piece of rope. "This is what you use to strangle people you don't like," he told them.
Mr Smith, 20, also from Congleton, denies murdering Claire last June. The prosecution said he tried desperately to hide the crime. He begged his mother to wash his clothes and was later seen back at the river in a pair of green waders.
The court was told Claire had experienced a difficult start in life, suffering sexual abuse before finding the security of a foster family and settling down at a new school. It was an "ironic tragedy", said Patrick Harrington QC, for the prosecution, that Claire should have died a "brutal, senseless death" just as she had "reached the threshold where the future was looking better than the past".
Claire had waved goodbye to her mother at about 8.20am. She was late leaving for school, having just returned from a school trip to France.
One of Claire's schoolfriends had walked behind her along the riverbank to school, and soon afterwards saw her sitting in a playing field talking to Mr Smith. Claire did not seem at all distressed, but Mr Smith, the boy said, was carrying a gun. Claire knew Mr Smith, who spent much of his time in the woods around the River Dane, fishing and firing air guns.
It is uncertain whether Mr Smith coerced her out of the field. But after strangling her, possibly with her own school tie, he disposed of her body in the fast-flowing river, said the prosecution. Claire's body was found five days later.
Mr Smith had immediately changed out of the tracksuit bottoms he was wearing, which were later found soaking wet, the prosecution told the court. Mr Smith said he had recently washed them. The prosecution said the water was from the River Dane.
The trial continues today.Reuse content