"What you've got with a search like this is a jigsaw puzzle, but you don't know what the picture is".

Mike Swindells knows more than most about putting together the puzzle behind a missing persons case - as a consultant to the Forensic Search Advisory Group, he has lent his expertise to forces across the UK.

"The first thing you have to do is find out as much as you can about the missing child, whether they regularly go missing, common sense things really.

"Then you have to look at the time since they were last seen, how far they could have gone, did they have any money on them and if so, how far that would get them.

Balancing act

"You would also look at CCTV pictures from the area, and from nearby train stations and bus stops."

The search would then concentrate on the area where the missing people were last seen, with police, often helped by the public, combing the area for clues.

Mr Swindells said there was a balancing act involved when the public helped in searches.

He said: "Sometimes I would say, if you had a murder in a house would you let the public wander through the house? In the same way, members of the public, who aren't trained, can end up trampling over evidence rather than finding it.

It is important to get the public involved, to show you are doing something

Mike Swindells

"But it is also important to get the public involved, to show you are doing something."

As with the missing girls in Cambridgeshire, a search can involve combing through e-mails and text messages as much as through the streets.

Forensic engineers will search through messages the missing children may have sent in the hope of finding clues to where they may have gone.

"More often than not with these cases, you find that what has happened is that the child has stayed over with a friend without telling someone, or they have decided to go to Blackpool for instance.

Safe and well

"It's often said that if the person you are looking for is not found within 48 hours then it is likely you are looking at an abduction or murder, but that is not always the case.

"I have known cases where children have been missing for weeks, and then turned up safe and well."

He said that the case in Cambridgeshire was one where he would probably be optimistic.

"If I was the search manager I would be happy - obviously not happy about the fact the girls are missing - but happy about the fact that there are two of them.

"Spontaneous abductions are very few and far between, and with two kids it would be very difficult to do."