UK detectives searching for missing 10-year-olds Holly Wells and Jessica Chapman say they are increasingly concerned they may have been abducted.
Detective Superintendent David Hankins, the officer leading the hunt, says police fears for the safety of the girls are being "raised by the hour".
Holly and Jessica vanished from Holly's home in Soham, Cambridgeshire, on Sunday evening.
Mr Hankins told the BBC: "Our concerns and our fears are raised by the hour. We are increasingly having to look at the possibility that the girls have been abducted and that this will be a criminal investigation."
Detectives who led the hunt for murdered schoolgirl Sarah Payne will be helping search for the youngsters.
Mr Hankins said calling in Sussex Police officers from the Sarah Payne inquiry had been "the most harrowing thing imaginable for the family".
He went on: "We have got our fingers crossed and hope and pray that the girls are somewhere safe, but we have got to be realistic.
"We have to look at the possibility that they have not been seen for three days and could have been abducted."
Officers are also to begin searching waterways, he added.
PA