Police say they have "real optimism" about their investigation into the disappearance of Leah Croucher, after tracing a dog walker photographed near a lake.
Leah was 19 when she was last seen heading to work in Milton Keynes on 15 February 2019.
Detectives last month launched an appeal to find a woman photographed walking her dog by Furzton Lake, as they ramped up the investigation again on the third anniversary of her last sighting.
Thames Valley Police said the woman had now come forward and contacted officers.
As a result, they have re-released more photographs in the hope of tracking down other people who may be able to help their inquiry into what happened to Leah.
She was reported missing from Emerson Valley, Milton Keynes on 15 February 2019, by her family who had last seen her at 10pm the day before, on Valentine’s Day.
The last confirmed sighting was at 8.16am on CCTV in Buzzacott Lane, Furzton, on the morning of 15 February, and the last activity on her phone was just after 8.30am that day.
One of the images re-released by police shows the figure of a person dressed in black, who has never been identified and whom police say may be Leah on the day she disappeared.
The image also features other members of the public, and police are now trying to make contact with them.
Detective Chief Inspector Andy Howard said: “The images we are re-releasing today are not of high quality, however, we are hoping that someone will recognise themselves as being in the picture.
“I would ask you to cast your minds back to Friday 15 February 2019, and if you tended to frequent the area of Furzton Lake, and believe you were in the area that morning, please take a close look at these images."
They were taken the day after Valentine’s Day 2019, and just before the school half-term holidays.
“Is it possible that the images have captured you?" said Det Ch Insp Howard. “I am very keen to hear from anybody in the group of people in this image, and particularly keen to identify and speak to the figure dressed in black. It may be that you regularly walked in this area back in 2019 and that you will recognise yourselves."
“You may have that vital clue or piece of information.
“The photograph of the person dressed in black was taken at 10.51am on the morning of 15 February 2019.
“It is not possible from the image alone to say whether the person dressed in black is Leah, but we do have witnesses in the area of the lake at around this time who describe seeing a young female in black clothing who is described as appearing distracted or upset, and possibly using her phone."
Last month, Leah's parents made an emotional appeal for information so that they could find out if their daughter, who would now be 22, was still alive or not.
Claire Croucher said: "I need that one person that has some information to come forward so I can get my daughter back and give her a funeral, get her ashes and put them in a necklace next to her brother.
"I just need the closure because not knowing is what's driving me insane."
Det Ch Insp Howard said the force remained "absolutely committed to finding Leah and seeking answers for her family".
“It has been more than three years now since Leah disappeared without trace, and despite extensive investigations, we still do not know where she went after the sighting on CCTV in Buzzacott Lane," he said.
“It was very encouraging that releasing these images on the third anniversary led to the female dog walker contacting the force. Although this did not provide us with any new leads, it gives me real optimism that further information is out there that can assist us in finding Leah."
A £20,000 reward has been offered for information that helps find Leah, and is still available.
Det Ch Insp Howard added: “The response from the public since Leah went missing has been incredible and I know that Leah’s case remains very much at the forefront of the minds of the residents of Milton Keynes."