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.
Detective Chief Inspector Andy Redwood said police "continue to believe that there is a possibility that Madeleine is alive."