BRITISH police are set to begin exhaustive interviews with anyone who may have come into contact with Gerry and Kate McCann since their schooldays as their Portuguese colleagues await the findings of crucial forensic tests.
The results of checks on hair and bodily fluid samples found in the family's hire car are expected to be revealed as early as tomorrow and will be vital in deciding which direction the inquiry takes next.
If they do appear to link the McCanns to the disappearance of their daughter, Madeleine, then hundreds of people can expect to be quizzed by detectives across the UK.
Portuguese officials have already made an official request for their British colleagues to start their questioning as soon as the results are released.
Close family and friends of the two doctors have already been interviewed by police, but that number will increase dramatically if the forensic findings implicate the McCanns.
Madeleine vanished from a holiday villa in the Portuguese resort of Praia da Luz on May 3, prompting a global hunt.
Initially, it was believed the four-year-old had been snatched by a paedophile, but in some quarters suspicion started to focus on her parents.
Last month, detectives in Portugal gave the couple 'arguido' - or official suspect - status, something which remains to this day.
They have vehemently denied any involvement in their daughter's disappearance and have launched a high-profile campaign to clear their name.
One British police source said: "Basically, if things do appear to be pointing towards the family then it will be a case of going back to basics and starting from scratch.
"Everyone who may have had any dealings with either Gerry or Kate as they grew up may potentially hold vital information about them which could prove vital to the inquiry."
The Portuguese have already issued a formal application, called a commission rogatoire, which is used to obtain evidence from one European jurisdiction for use in another.
This was done recently via Europol in The Hague and is a standard procedure in any trans-European investigation.
If more extensive inquiries are required, officers from at least four British police forces, including Strathclyde and Tayside, will become involved in the hunt.
Gerry McCann was born in Glasgow and attended Holyrood Secondary School on the city's Southside before stating his medical studies at Glasgow University.
It was whilst on a placement at a nearby infirmary that he met his future wife, a Liverpudlian who studied at the University of Dundee. The couple now live in Leicestershire with their two-year-old twins, Sean and Amelie.
The forensic samples were discovered in a Renault Scenic, which was hired by the McCanns 25 days after Madeleine vanished.
The girl's parents claim there is an entirely innocent explanation for the presence of the hair and fluids, but detectives in Portugal believe it backs up their story that the couple were somehow involved in their daughter's disappearance.