(CNN) -- Detectives investigating the disappearance of Madeleine McCann in Portugal are looking at six more cases where a male intruder entered holiday villas where children were staying, police said Wednesday.
The development means there are now 18 potentially linked crimes at resorts near where the 3-year-old vanished in June 2007, London's Metropolitan Police said.
Children were sexually assaulted by the intruder in half of those cases.
Madeleine's disappearance while on a family vacation in the Portuguese resort town of Praia da Luz prompted headlines worldwide -- and remains a mystery.
The six new cases have come to light thanks to an appeal for information last month that prompted more than 500 calls from the public, a police statement said.
"Five involve sexual assaults on children and one was a 'near miss,' " it said.
"Of particular interest to the team is that one of the new sexual assaults took place in Praia da Luz in 2005."
All but one of the six offenses had been reported to the Portuguese police at the time of the Madeleine's disappearance, the Metropolitan Police said.
"These new cases are similar to a number of the originally identified (12 offenses) whereby a male intruder has gained access to holiday villas occupied by UK families in the Western Algarve," the force said.
Of the 18 potentially linked incidents, five were in Carvoeiro, nine in the Albufeira district, three in Praia da Luz and one in Vilamoura. The offenses in Praia da Luz occurred between 2005 and 2010.
When police appealed for information last month, they said witnesses described the intruder as being tanned with short, dark, unkempt hair.
The man was said to have spoken in English with an accent, and his voice was described as slow, or possibly slurred.
No new information about the suspect was given Wednesday.
In October, new police sketches were released of potential suspects in the case, and UK police appeared in a television appeal for information.
The program prompted a flurry of tips, and police in Portugal announced they would reopen the case.
Since then, the Portuguese investigation has run parallel to the British one.
Detectives previously said they were investigating a spike in break-ins in the area in the weeks before Madeleine disappeared, two in the same block where her family was staying.
Neither her parents, Kate and Gerry McCann, nor the detectives investigating her case have given up on one day finding the little girl from Leicestershire, England.
CNN's Claudia Rebaza and Marie-Louise Gumuchian contributed to this report.