Madeleine disappeared from her room at a holiday resort in the Algarve, Portugal, on May 3, 2007, as her parents dined at a nearby restaurant, sparking a global manhunt and transfixing the world's media.
Speaking to the Evening Standard newspaper, Detective Chief Superintendent Hamish Campbell, head of Scotland Yard's Homicide and Serious Crime Command, said: "There are a lot of people of interest," without saying how many.
"There are people who could be properly explored further, if only to be eliminated."
Despite reported sightings the world over, the fate of Madeleine McCann remains a mystery.
"Kate and Gerry remain very, very pleased with the work that Scotland Yard are doing and have been encouraged by Operation Grange from the day it began," the McCanns' spokesman Clarence Mitchell was quoted as saying by the British media.
British Prime Minister David Cameron ordered a new investigation by London police after the McCanns wrote to him in 2011 saying neither British nor Portuguese authorities had done enough to try to find their daughter.