At first it was believed that Stacey Queripel had died accidentally when her necklaces became tangled on branches in woods near her home in Bracknell.
A police spokesman said: 'Following a further forensic examination by the Home Office pathologist, we are now treating Stacey's death as a murder investigation.'
Since the original post-mortem examination, more marks have come out on the body which showed Stacey was strangled. A second examination was carried out yesterday.
Her body was found lying in undergrowth by a path in the South Hill Park arts centre a few hours after she crept out of her home in a second-floor flat on the Birch Hill estate. The initial post-mortem examination concluded that she had died from asphyxia.
Stacey disappeared last Sunday after asking to go to bed early and was last seen by her mother Gilliane at 5.30pm.