British police say they have launched a full investigation into the disappearance of Madeleine McCann, and want to trace 38 "persons of interest" in the case.

Detectives say it's possible that Madeleine, who vanished from a Portuguese holiday resort six years ago, is still alive.

Scotland Yard said Thursday that 12 of the individuals of interest are British, and the rest from a variety of European countries. The force says it is working with governments across the Continent to find out more.

McCann vanished from a vacation home in Portugal's Algarve region on May 3, 2007, days before her fourth birthday. The case has generated intense media interest.

Portuguese police closed the case in 2008 due to lack of evidence, but Scotland Yard started a review in 2011 that eventually led to a full investigation.

"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," Det. Chief Insp. Andy Redwood said.

"We continue to believe that there is a possibility that Madeleine is alive."

The McCanns issued a statement welcoming the new development.

"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," said Clarence Mitchell, spokesman for Madeleine's parents Kate and Gerry McCann.

Ted Davis, senior investigator at the Missing Children Society of Canada, a Calgary-based organization that assists police and families in search of missing children, said the optimism of police is not unusual in missing persons investigations. 

"I don’t ever lose the thought that the person I’m looking for is alive, because there have been so many cases where kids and people have gone missing and they come back. It’s miraculous."

Davis said, however, that he has mixed feelings about the intense media coverage of the McCann case.

"There’s more than Madeleine McCann in this world that’s missing," he said. "I’d like to see her return alive and well, as well as [the children in] 30 different files we have ... and would like to see the publicity on it, but it just doesn’t come that way."

With files from CBC News