Her body was found on Saturday night after guests at a wedding reception left the Merchistoun Hall, in the Horndean area of the city, to take some air.
Det Supt Douglas Quade, leading almost 40 Hampshire officers in the murder hunt, said: 'Her clothing had been interfered with and we believe the motive for her murder may have been sexual.'
Helen was last seen at 11pm on Friday when she left her home in Horndean. Mr Quade said he believed there was no link between the wedding and the murder.
Helen lived with her mother and elder brother. He said they knew she was going out on Friday night, but did not know her plans. Her mother contacted the police after hearing appeals on local radio for help in identifying the girl.
'We're anxious to know where she went, who she met, and what she did,' Mr Quade said. Police were hoping to interview her boyfriend and friends from the local Purbrook Place School.
Merchistoun Hall is used as the Horndean community centre and is set in four-and-a-half acres of parkland crossed by paths. The woodland area where Helen was found is about two to three minutes' walk from her home.
The area was sealed off yesterday as police carried out a fingertip search of the scene and house- to-house inquiries nearby. There was no sign of a struggle and Mr Quade said he believed she had been murdered on the spot. Medical evidence suggested she had been killed at least eight hours before she was found and possibly only minutes after she left home.
A sex attacker wanted for the rape of an Oxford University student was feared to have struck again after a 27-year-old woman scientist was attacked as she walked home from a party in the city early on Saturday. She was dragged her into an alleyway where her hands were tied and she was assaulted.
Police said there were similarities with the rape of a 20-year-old student from Somerville College in public lavatories two months ago.
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