Detective Superintendent David Beck, the officer leading the hunt for the Holly Wells and Jessica Chapman, is to set up a telephone hotline for the abductor to call him in person "one-to-one" with a deadline of midnight on Thursday.
Mr Beck said: "It is giving these people, if they are more than one, an opportunity to have a one-to-one direct link contact with me before midnight tomorrow."
Mr Beck said he still believed the children were alive but said that a taxi driver's sighting of a green car, thought to be central to the abduction inquiry, had been timed to shortly after 6pm, not 7pm as the driver had reported.
The girls were sighted around the town of Soham after 6pm.