Detectives have launched a fresh search for missing Leicestershire youngster Madeleine McCann.
Met officers made the announcement at 2pm today.
They said they had identified 38 "persons of interest" who they wanted to speak to in relation to the suspected abduction of Madeleine in the resort of Praia da Luz in Portugal in 2007.
Speaking to the media, Detective Chief Inspector Andy Redwood, who is heading Operation Grange, said: "There is no clear, definitive proof that Madeleine McCann is dead.
"On that basis I genuinely believe there is a possibility she is still alive. And so I would like to ask the public to continue to look for her."
Parents Kate and Gerry McCann, from Rothley, have welcomed the new hunt for their daughter.
Clarence Mitchell, spokesman for the McCanns, said: "Kate and Gerry warmly welcome the shift in the Met's emphasis from review to investigation.
"It is clearly a big step forward in establishing what happened and, hopefully, towards bringing whoever is responsible for Madeleine's abduction to justice."
The new investigation will focus on 38 people from five countries who police believe were in Portugal at the time of Madeleine's disappearance. Twelve of those are British nationals.
The latest probe comes as a result of a two-year review which has analysed evidence from material collected by British and Portuguese police and numerous private investigators.
Detectives have scrutinised some 30,500 documents which they say have "generated new findings and new witness evidence".
A spokesman for the Met said: "In the absence of any clear evidence to the contrary we maintain our belief that Madeleine may still be alive."
He added: "We are satisfied that our review has now progressed to a position where we have identified 38 persons of interest. These individuals are from a number of European countries and we are now at an advanced stage of dialogue with each country."
Officers, working under Operation Grange, are aiming to travel to each of the countries, where they hope to pursue the new lines of inquiry.
They are also waiting for confirmation from the Portuguese authorities that a small team is present in Portugal whilst the inquiries are undertaken.
DCI Redwood said: "We, and the Portuguese authorities, remain completely committed to finding out what happened to Madeleine, and everything we do is utterly focused on her best interests.
"We continue to believe that there is a possibility that Madeleine is alive.
"It is a positive step in our hunt for Madeleine that our understanding of the evidence has enabled us to shift from review to investigation.
"We have identified 38 persons of interest from a number of European countries. Twelve of those people are UK nationals who we believe were in Portugal at the time Madeleine disappeared.
"Our working relationship with the Portuguese police is positive and now that we have moved to investigation we are requesting further specific assistance through normal judicial routes.
"We remain in close contact with Kate and Gerry McCann and they are updated on our current position."
DCI Redwood said the appeal for information was ongoing and urged people to get in contact if they had not yet spoken to police.
He said: "If you were at the resort of Praia da Luz between April 28, and May 3, 2007, either on holiday or in residence in the resort during this period, particularly in the vicinity of the Ocean Club, and you have not been spoken to by police either here or in Portugal then please call us."
Call police on: 0800 096 1011.
For non-UK residents: +44 2071580 126.
Alternatively, contact Crimestoppers anonymously, on 0800 555 111.
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