COPS investigating the death of Leah Croucher have revealed their only suspect "changed his appearance" to avoid capture.
Leah's remains were found in October at a property in Milton Keynes, Buckinghamshire, three years after her disappearance.
Neil Maxwell is the prime suspect in the case and is the only suspect who has been named by police.
However, investigators have been unable to place him near the house at the time and have released an e-fit of how he is likely to have looked in an effort to find witnesses.
They believe he may have lost weight, changed his name and grown a beard as he tried to evade capture for other offences.
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Maxwell took his own life in 2019 after officer made 18 attempts to arrest him.
His body was found in a bin cupboard in a local park, with a handwritten note on the door reading: "Please do not come in. There is a dead body inside. Call the police."
The convicted sex offender had carried out maintenance work on the house where Leah, 19, was found from November 2018.
Maxwell was the only person to have keys to the property while the owners, who are not involved in the case, were away.
He was already wanted by police in relation to a sexual assault in Newport Pagnell, Buckinghamshire before being connected to Leah's case.
There are now fears Leah may have been held captive at the address for weeks before she was killed.
She was last seen on February 15, 2019, in Milton Keynes as she walked to work in a finance firm.
The teen told her family the day before she was meeting a friend but that never happened.
CCTV showed her walking down Buzzacott Lane in Furzton, believed to be in the direction of her work, at about 8.15am the next day.
The house at the centre of the murder probe is just 0.4miles from where Leah's last movements were captured.
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It is also near Furzton Lake, where the teen's phone left the network - consistent with it being destroyed - at 8.34am.
Tragically, Leah's brother Haydon Croucher, 24, ended his life in 2019 after being "tortured" by not knowing what happened to his sister.
Detective Chief Superintendent Ian Hunter of Thames Valley Police said: "I can confirm that Neil Maxwell remains the only nominated suspect at this point of the investigation, and our investigation continues to diligently gather sufficient evidence to establish the truth.
"Maxwell knew he was wanted in connection with a sexual assault investigation that occurred in Newport Pagnell in November 2018 and appears to have made extensive efforts to evade arrest by using false names, and we believe he stopped using his mobile phone and his vehicles. He is likely to have known that he would be returning to prison if he was arrested and convicted.
"The investigation has now also established that Maxwell is likely to have changed his appearance to avoid being recognised. He had grown a beard and may have lost weight.
"We have therefore generated a computer image of how Maxwell may have looked around the time Leah went missing in the hope that it may jog someone's memory. Did you see this person in or around Loxbeare Drive, Furzton, at any time between December 2018 and 20 April 2019?
"Or can you help us with obtaining further information about Neil Maxwell. Do you know where he was between December 2018 and 20 April 2019, what phone number he was using, what vehicle he was driving, who he was with or any other useful information that can help us establish the truth for Leah's family?"
Anyone with information is asked to contact the force on 101, quoting Operation Innsbruck.
Those who wish to report information anonymously can contact independent charity Crimestoppers on 0800 555 111.
