A Latvian builder and Alice Gross's internet activity have become two major lines of enquiry in the investigation into the 14-year-old's disappearance.
In a Crimewatch programme last night presenter Kirsty Young described what may have happened to Alice as a 'complete mystery'.
In a new twist detectives used the show to appeal to the public to help them trace missing Latvian Builder Arnis Zalkalns, who disappeared seven days after Alice from the same area.
Detective Chief Inspector Andy Chalmers ran through her last known movements showing CCTV footage of her walking past Hanwell train station and along the canal.
He said the last known contact from her was a text to her father at 3pm asking him when he was going to be home later that day.
DCI Chalmers said: "This is first and foremost a missing persons' enquiry. But obviously the longer she remains missing the greater our concerns.
"She spent the morning with her mother and then left her home in Hanwell for a walk on her own which was a normal activity. She walked through Hanwell and along the canal to Brentford where we think she did some shopping.
"She is captured on CCTV at 2:15pm at Brentford Lock and 45 minutes later she texted her father to ask what time he was going to be home. This is the last contact anyone had with her.
"41-year-old Arnis Zalkalns is missing. What we know is that he left for work in the early hours of the morning of September 4. He left home to go to work on his red mountain bike. He hasn't been seen since. His normal route would have taken him along the part of the canal where Alice went missing. There's no suggestion that he knew Alice but clearly he is someone who I need to speak to. He might have important information.
"Five days later we found her bag on the side of the river Brent between Hanwell Bridge and the canal. It contained her lunch box and her shoes but not her white 4S iphone. She was in connection with the internet throughout her walk but went off line sometime after 5pm and it has not be used since. I am keen to speak to anyone who might be in possession of that phone. I am not concerned with how they may have come into possession of it.
"Alice's internet history is an important line of enquiry. We need to speak to anyone who may have spoken to her through chat rooms or media sites."
Mr Zalkaln's disappearance has been described as being completely out of character. He has a partner and child in Ealing and is believed to have been working in the Isleworth area.
Alice's mother, Rosalind Hodgkiss, is also filmed in the programme. She said: "The last two weeks have been completely heart breaking. There is not one minute of the day that we don't think about Alice and where she is, what might have happened or why she might have gone missing. It is almost impossible to describe what that pain feels like."
