Police searching for Madeleine McCann are trying to trace six British cleaners working in Portugal when she vanished.
The group were said to have been in the Praia da Luz holiday complex when Madeleine – who would now be nine years old - disappeared in 2007.
A Met Police spokesman said speaking to individuals was part of its review, adding: "Officers are reviewing all of the material in relation to the disappearance of Madeleine McCann.
"This is routine procedure as part of the ongoing review."
The officers became aware of the six to eight British workers, said to have used a white van, after going over evidence from the original Portuguese investigation.
Now they are asking owners whose properties overlook the apartment where Madeleine stayed for information about the group.
One property owner had a letter from Scotland Yard which says officers are checking who was in the flats at the time.
Madeleine was nearly four when she disappeared in the Algarve as her parents dined at a tapas restaurant nearby. Her parents Kate and Gerry have campaigned tirelessly since to trace their daughter.