On the afternoon of December 12, 2000, a few days before her 16th birthday, Melbourne teenager Cherie Westell made a reverse phone call to a family member from a public phone box in Melbourne’s east, telling them she was catching the train home from the dentist.

In more than 20 years Cherie, a ward of the state, has never been seen or heard from again.

On Wednesday, Victoria Police announced a $1m reward for anyone with information that could solve the mystery of Cherie’s disappearance, with Detective Acting Inspector Tony Combridge declaring they “will find” those responsible.

Cherie was dropped off at Knox Dental Group in the Melbourne suburb of Wantirna South at about noon on the day she went missing before leaving at about 12.30pm.

At 1.58 that afternoon police said Cherie made a phone call from a public phone box on the corner of Selkirk Ave and Wantirna Rd, about 5km from the dentist, telling a family member she was catching a train home to Ringwood.

The call was the last known contact with Cherie.

A missing person’s report wasn’t filed for six days because the person trying to make the report, her earlier foster mother Francis, was told she was not an immediate family member.

Victoria Police said Cherie “most likely met with foul play,” while Inspector Combridge said he did not think the person responsible for the girl’s death was known to her, labelling it as the “worst-case scenario.”

On Wednesday, Cherie’s foster sister, Marg, said that within a few days they knew “something had happened to her”.

“She was a state ward but she wasn’t a runaway,” she said.

Her previous foster mother, Francis, told the media that Cherie was “confused” by the age of 14 owing to her living arrangements but insisted she was “very loving” and said her disappearance was “completely out of character”.

“The word closure doesn’t exist – but it would give us answers,” she said.

Inspector Combridge said “allegiances change” over the years and “there are a lot of people of interest to us.”

He said he did not know if Cherie ever made it to the train station.

A coronial inquest in 2003 also found that it was likely Cherie had died.

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