British detectives open new investigation after reviewing all evidence into disappearance of three-year-old from Portugal

British detectives are targeting 38 potential suspects from five countries as they open a fresh criminal investigation into the Madeleine McCann case six years after she went missing from her family's holiday apartment in Portugal.

A formal request for assistance has been sent to the Portuguese authorities as officers from the Metropolitan police pursue what senior detectives involved say are "new theories and new evidence" about what happened to the three-year-old.

Detectives are preparing to be based in Portugal once the authorities there approve, as anticipated, the request to provide assistance to their investigation.

Officers are also working with police and authorities in three other European countries where potential suspects are based, with the aim of asking for their future assistance with inquiries and arrests.

Scotland Yard asked for media restraint in the coming weeks and months as it began what is being seen as the last chance to learn what happened to Madeleine. Officers believe there is a chance she is still alive.

The 38 "persons of interest" who emerged from a two-year review of the evidence by Met officers come from five countries, including Portugal and the UK. They include 12 British nationals, some based in Portugal and some in the UK.

Madeleine McCann on the day she disappeared.

Detective Chief Inspector Andy Redwood, who is leading the inquiry, has spent the past two years reviewing the vast amount of evidence from the Portuguese criminal investigation, Leicestershire police – the McCanns' home force – and information from seven private detectives hired by Madeleine's parents, Kate and Gerry.

He said the Met team was in a unique position, having put all the evidence – amounting to 30,500 documents – together, and the criminal investigation would now pursue thousands of leads and key persons of interest.

"I believe that there is a possibility that Madeleine is alive," he said. "The review has given us new thinking, new theories, new evidence and new witnesses to pursue. It is a positive step in our hunt for Madeleine that our understanding of the evidence has enabled us to shift from a review to an investigation."

It is understood that the review has also thrown up forensic leads to examine.

A spokesman for the McCanns, Clarence Mitchell, said: "Kate and Gerry warmly welcome this shift in the Metropolitan police emphasis. They see it as a huge step forward in establishing what happened and hope that it will lead to bringing to justice whoever was responsible for Madeleine's abduction."

Detectives are at an early stage of the new investigation, known as Operation Grange, and arrests are not expected in the immediate future.

The original Portuguese investigation was shelved in 2008 but, after a request from the McCanns, David Cameron asked the Met to carry out a review, which began in 2011.

The £5m review – paid for by the Home Office – involved getting access to and translating tens of thousands of documents, and has created 3,800 leads which detectives will pursue.

The Ocean Club in Praia da Luz where the McCann family was staying when Madeleine disappeared. Photograph: Linda Nylind

Officers – who are two-thirds of the way through completing their review – have been to Portugal 16 times and shared their findings with the police and the judicial authorities as new information emerges. Prosecutors from London have also travelled to Portugal as part of negotiations to pave the way for the opening of the British investigation.

Madeleine was nearly four when she went missing from her family's holiday apartment in Praia da Luz in May 2007 as her parents dined with friends nearby.

The 38 potential suspects being examined are understood to include known child sex offenders who were in the Algarve at the time of her disappearance.

Redwood said none of the individuals was connected to Madeleine's family or friends who were with her parents on holiday at the time. The Met team's work leads them to believe Madeleine was abducted in a criminal act by a stranger.

Redwood said: "We remain in close contact with Kate and Gerry McCann and they are updated on our current position."

• This article was amended on 5 July 2013. The headline on the original described the 38 "persons of interest" as suspects; they are potential suspects.