Forensic tests are being carried out on suspected traces of blood found in the apartment where Madeleine McCann was last seen on 3 May, the BBC has learnt.
Madeleine, four, has been missing since 3 May
It is understood the find was made during a search - with at least one British sniffer dog - of the McCanns' now-vacant apartment in Praia da Luz.
If tests confirm the find, police will look for a DNA match with Madeleine.
A new search of the home of suspect Robert Murat, 33, in the Portuguese resort, found no new evidence.
Madeleine, four, from Rothley, Leicestershire, went missing from the Algarve holiday apartment while her parents ate in a nearby restaurant.
It is believed detectives returned to the McCanns' holiday apartment last week with at least one highly-specialised sniffer dog from Britain.
Inside, they found traces of what is thought to be blood but investigators cannot be 100% certain, a Portuguese police source told BBC News.
On Monday, detectives examined a car and a van used by the only declared suspect, Mr Murat.
Last night, the vehicles were returned to the Murat family villa.
The property is owned by Mr Murat's mother.
Publicly the police have not commented on the results of the weekend search but the BBC understands it revealed nothing to link Mr Murat to Madeleine.
Mr Murat was declared a suspect 10 days after Madeleine was last seen, on 3 May, but has strenuously denied any involvement in her disappearance. The property was searched then too.
The BBC understands British police were invited to review key aspects of the case by their Portuguese counterparts.
Police are also investigating a possible sighting of Madeleine at a restaurant in Tongeren, Belgium.
On Sunday, Madeleine's parents Kate and Gerry McCann attended Mass at a local Catholic church in Praia da Luz.
Mrs McCann has spoken of her regret at leaving her children alone in their holiday apartment while she and her husband Gerry had dinner at a nearby restaurant on the night Madeleine disappeared.
"We are just so desperately sorry. Every hour now, I still question, 'Why did I think that was safe?'"
The McCanns have backed a scheme aimed at letting parents and staff know what steps to take as soon as a child goes missing on holiday.
Code Madeleine is based on Code Adam in the US, named after abducted six-year-old Adam Walsh.