A commuter seen talking to missing Kent teenager Megan Doran at Charing Cross station could hold vital clues to her whereabouts, police have said.
CCTV footage showed Megan Doran (right) talking to a woman
CCTV shows the 13-year-old arriving at the station at 1108 BST on 23 April.
Officers say she is seen approaching a smartly-dressed woman aged between 40 and 50, possibly to ask for directions.
Det Ch Insp Dean Barnes said: "We urgently need to know what they talked about. Was she asking for directions. If so, where was she wanting to go?"
He added: "We believe this woman is probably a commuter and feel certain she would remember her conversation with Megan, who has a strong South African accent."
CCTV footage shows Megan walking through the ticket barrier and going to the ladies' toilets at the central London station, before coming out and approaching the woman.
Officers already know Megan spent Thursday night with a friend, whose mother dropped her at Tunbridge Wells train station on Friday morning.
Megan last spoke to her mother that afternoon, when she said she was with friends in a clothes shop in Oxford Street, London.
Megan is described as a "popular" pupil at her school in Tonbridge
She was told to be home by 1800 BST, but never returned.
Police have studied CCTV from Tunbridge Wells station and the Dorothy Perkins store.
Detectives also said the schoolgirl had visited internet chatrooms where she spoke with a variety of people and
officers are working to establish who they are.
Megan, who moved from South Africa with her family three years ago, is a popular pupil at Tonbridge Grammar School, where head teacher Wendy Carey said pupils and staff were "desperate" for good news.
She said: "Megan is a confident young lady; very positive and outgoing and has a wide circle of friends.
"She is clever and a very good all-round student. She is interested in the arts and drama. She is the sort of pupil that schools love to have."
The head teacher said pupils at the school had received guidance on the potential dangers and risks of internet chatrooms.
Megan is described as 5ft 3in tall and was last seen wearing light blue jeans and a grey jersey top.
Appeals to Megan have stressed she is not in trouble and her parents want her to come home.