The detective leading the hunt for two missing 10-year-old girls has made a direct appeal to "any possible abductor" who might be holding the British youngsters.
Acting Detective Superintendent David Beck said he remained optimistic that Holly Wells and Jessica Chapman were alive and well.
He urged anyone who might be holding them: "Get in touch with somebody so that we can get the girls home safely."
Holly and Jessica, both from Soham, Cambridgeshire, vanished on Sunday afternoon after spending much of the day playing at Holly's home.
"We remain optimistic that Holly and Jessica are alive and well," Mr Beck told reporters outside Cambridgeshire Police's headquarters in Huntingdon.
"However, as the days continue we must consider the possibility that they are being held against their will.
Police have released CCTV footage of the youngsters in the town centre shortly before they disappeared in Cambridgeshire.
The pictures show the two youngsters, dressed in identical Manchester United shirts, walking past a sports centre moments after they were last seen at Holly's home in Soham on Sunday evening.
Det Supt David Hankins said: "It clearly shows four people as well as the two girls, two men and two women. They are walking from the sports centre towards the car park".
A newspaper group has offered a £1 million sterling reward for information about the two 10-year-olds. Express Newspapers said it would give the reward to anyone who provided information leading to the safe return of the girls or the arrest and conviction of anyone found to have abducted them.
The reward offer followed an earlier one of £150,000 sterling from the Sunand the News of the World, part of the News Corporation group.