Police searching for 14-year-old Alice Gross have revealed that a missing Latvian man has a conviction for murder and is now their prime suspect.
Builder Arnis Zalkalns, 41, served seven years in prison for murder in Latvia, after being convicted in 1998.
Police say he was arrested in London in 2009 for indecently assaulting a 14-year-old girl but no further action was taken.
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Mr Zalkalns - who lives in Ealing and has a partner and child - disappeared a week after Alice went missing.
He normally used the Grand Union Canal route to get to work but on September 3, he failed to show up - which police described as 'completely out of character'.
Alice, from Hanwell, West London, was last seen on CCTV cameras on 28 August by the same canal route in west London.
The Latvian was seen on CCTV on 28 August cycling along Brentford Lock at 4pm, some 15 minutes after Alice walked that route.
Detectives believe he is likely to have come across Alice as they were both going in the same direction, north along the canal towpath.
A reward of up to £20,000 is being offered for anyone who has information that leads detectives to find Alice.
Detective Superintendent Carl Mehta, of the Met's homicide and major crime unit, said: "On Thursday 28 August Arnis Zalkalns left work, in Isleworth, at about 3.30pm on his bike. Did you see him that afternoon? Either on his own, or with a teenage girl?
"I also need to hear from anyone who knew Arnis whom we have not yet spoken to. His family and work colleagues have stated he was behaving normally in both the days before and after Alice's disappearance. They are deeply concerned about his disappearance.
"Arnis has not been seen since Wednesday 3 September. I want anyone who may have seen him after that date, or who may have helped him by giving him money or somewhere to stay, to call and speak to us. It is really important that we trace Arnis and speak to him about Alice's disappearance.
"Three weeks ago today Alice left home as usual. She has not been seen since 4.26pm, when she walked along Trumpers Way.
"So far the public have given us great support, but there may still be people out there who were walking in the area Alice was on that Thursday who have not yet spoken to us. If you were and you saw anything that could help progress our investigation - please call.
"Alice used social networking sites, Facebook, Twitter and Ask.fm. I'm asking anyone who may have been talking to Alice on these sites in the run up to the day she disappeared to speak to us."
In an interview with Crimewatch this week, Alice's mum, Rosalind Hodgkiss encouraged anyone with information about the teenager's whereabouts to 'come forward to the police and get her home because that's where she belongs and she needs to be here with us'.
She said: "There's not a moment of the day that you don't think about Alice and where she is, what might have happened or why she might have gone missing.
"It's almost impossible to describe what that pain feels like but we just want her to know: please Alice, if you're out there, come home."