Detectives investigating the disappearance of British girl Madeleine McCann in Portugal said they were looking for a lone intruder who sexually abused five girls after breaking into holiday villas.
The tanned, dark-haired man is suspected of breaking into 12 properties where British families were staying in the Algarve between 2004 and 2010, the Metropolitan Police said.
In four of the incidents, girls aged between seven and 10 were sexually assaulted while they were in their beds. In one case two girls were assaulted in the same villa.
The attacks happened between 2004 and 2006, before Madeleine vanished from a holiday apartment in 2007, days before her fourth birthday.
Her parents were dining with friends at a nearby restaurant at the time.
Police believe there are striking similarities between the break-ins. In most cases there were no signs of forced entry to the property and the intruder appeared between 2:00am and 5:00am. The man, who spoke English with a foreign accent, remained calm even when he was disturbed.
On two occasions the noise of a refuse collection lorry could be heard nearby.
Two of the break-ins were in the resort of Praia da Luz, where Madeleine McCann disappeared.
Detective Chief Inspector Andy Redwood said tracing the man, who is said to have "an unhealthy interest in young white female children", was one of his priority lines of inquiry.
"These offences are very serious and no one has been charged in connection with them," Redwood said.
"We also need to eliminate this man from our enquiries and ascertain whether these offences are linked to Madeleine's disappearance."
His team, who are working with Portuguese authorities, currently has 38 people classed as "persons of interest" and is also sifting through details of 530 known sex offenders whose whereabouts cannot be accounted for.
Of those 59 are classed as high priority, and some are British.
Portuguese authorities closed their investigation into Madeleine's disappearance in 2008, but the Metropolitan Police spent two years reviewing the evidence at the British government's request and opened their own probe in July last year.
The girl's parents Gerry and Kate McCann have never given up their campaign to find Madeleine.