A dog walker is being urged to come forward by detectives investigating a sex attack under a disused railway bridge.
The victim, who is in her early 20s, was severely distressed by the attack at midnight on Christmas Eve but was otherwise uninjured.
She was targeted as she walked under the bridge on the edge of Malling Field, near the River Ouse, in Lewes.
Police are keen to speak to a man who was walking his dog at the time of the alleged assault.
A police source said: "We understand that there was an elderly man, walking a large white dog, who may have stopped and spoken to the couple without realising what was happening.
"We do not know if he is a witness but we do urgently need him to contact the police."
A local man in his 20s has been arrested and released on police bail pending further inquiries. Anyone with information should contact on 0845 6070999.
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