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Holly Wells and Jessica Chapman are best friends, both from Soham, and were last seen at about 1830 BST (1730 GMT) last night.
They had been at a family barbecue at Holly's house, and were thought to be playing in Holly's bedroom, but when Holly's parents went to check on the girls at 2030 BST (1930 GMT) they had vanished.
A police helicopter was carrying out an extensive search of the area on Monday along with personnel from the nearby United States Air Force based at RAF Mildenhall, Suffolk, and 100 local volunteers.
Inspector Simon Causer, from Cambridgeshire Police, said, "We are extremely concerned.
"There appears to be no reason why either girl would go off and both parents are extremely worried."
The girls had spent the day playing together and had returned to Holly's home in Red House Gardens, Soham, in the late afternoon.
Since then a member of the public has reported seeing them walking along nearby Sand Street at about 1830 BST (1730 GMT).
"Quite clearly they are 10-year-old girls who have been missing all night and we are genuinely concerned for their safety," Inspector Causer continued.
"They are both small and slightly built and they are both wearing Manchester United football shirts."
Police say that Jessica had a mobile phone with her when she went missing and engineers have traced it to an area of Cambridgeshire surrounding Soham.
Attempts were made to ring it, but police believe the battery is dead or the credits have run out.
Both girls are described as being white, about 4ft 6in (1.37m) tall and slim.
Jessica is tanned and has shoulder-length brown hair while Holly is fair and has blonde hair.
The girls are in the same class at St Andrew's primary school in Soham.
Jessica and Holly's disappearance triggered one of Britain's biggest manhunts, involving more than 400 officers from 21 forces.
More than 14,000 calls were made to police in the first two weeks of the investigation.
The bodies of Holly Wells and Jessica Chapman were found almost two weeks after they went missing, hidden in a ditch near RAF Lakenheath airbase in Suffolk.
On the same day, school caretaker Ian Huntley, 28, and his girlfriend Maxine Carr, 25, who had been a teaching assistant in the girls' class, were arrested in connection with the girls' deaths.
Huntley was convicted of both murders on 17 December 2003.
Carr was acquitted of helping him, but convicted of conspiring to pervert the course of justice. She was released in May 2004.
A series of bureaucratic slip-ups had allowed Huntley to be employed working with children despite a series of previous accusations of rape and underage sex.
The Bichard Inquiry into how this could have happened resulted in a damning indictment of the Humberside police force.
The Chief Constable, David Westwood, defied the Home Secretary and refused to resign. He was supported by his police authority, but eventually agreed to take early retirement.
Huntley's house, where Holly and Jessica were murdered, was demolished in April 2004.
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