The new suspect in the Madeleine McCann case has also been investigated over the disappearance of a German girl, according to German media reports.
The 43-year-old German man, named in reports as Christian B, is currently serving a prison sentence.
He is believed to have been in the area where Madeleine, three, went missing while on holiday in Portugal in 2007.
A source told the BBC an investigation into the suspect began after a tip-off in Germany in 2017.
Meanwhile, a spokesman for the McCanns said they would be “encouraged” by the response to the latest appeal for information.
The UK’s Metropolitan Police said it had received more than 270 calls and emails since revealing details of the new suspect on Wednesday.
German prosecutors have said they are assuming Madeleine is dead and that the suspect is being investigated on suspicion of murder.
A number of German media reports said the suspect had been investigated over the disappearance of a five-year-old German girl – named only as Inga.
She went missing from a family party in Saxony-Anhalt on 2 May 2015 and has never been found.
Police have refused to confirm the investigation or comment on a report which says officers searched an area of nearby land belonging to Christian B in February 2016.
A senior judicial source in Portugal has told the BBC the joint investigation into the new suspect began after a tip-off in Germany in 2017.
The source said German investigators informed their Portuguese counterparts and British officials three years ago that they received a tip-off from a friend of Christian B, after the suspect had made a “disturbing comment” in a bar in Germany, as they were watching TV news coverage of the 10th anniversary of Madeleine going missing.
Since that time, the official said Portuguese police have been making house-to-house enquiries speaking to those that knew him between 1996 and 2007.
While he was in Germany, he was convicted of the rape of a 72-year-old woman which took place in Praia da Luz in 2005.
The official added that the investigation linking Christian B to the Madeleine McCann case was now reliant on the help of the public to find clear evidence.
On Thursday, German prosecutors said the suspect was “a sexual predator who has already been convicted of crimes against little girls” and was “serving a long sentence”.
Hans Christian Wolters, from the Braunschweig Public Prosecutor’s Office, said the suspect was regularly living in the Algarve between 1995 and 2007 and had jobs in the area, including in catering, but also committed burglaries in hotels and dealt drugs.
The Met Police, who are working with their German and Portuguese counterparts, said the case remained a “missing persons” investigation in the UK because it does not have “definitive evidence” as to whether Madeleine is alive or not.
Clarence Mitchell, a spokesman for Madeleine’s family, said her parents, Kate and Gerry McCann, from Rothley, Leicestershire, are “trying to maintain as normal a life as possible” and awaiting updates from police.
“Two-hundred-and-seventy calls and emails isn’t a bad result, given it was 13 years ago,” he said.
“They certainly will be encouraged to know the appeal is yielding results already and hopefully within that there will be crucial bits of information the police can act upon.”
Det Ch Insp Mark Cranwell, who is leading the Met’s investigation, known as Operation Grange, said he was “pleased” with information coming in after receiving more than 270 calls and emails by 16:00 GMT on Thursday.
Police have also released photos of two vehicles – a VW camper van and a Jaguar car – which are believed to be linked to the man, as well as a house in Portugal.
The day after Madeleine vanished in 2007, the suspect transferred the Jaguar to someone else’s name.
He was in the Praia da Luz area where the McCann family was staying when she disappeared and received a phone call at 19:32, which ended at 20:02. Madeleine is believed to have disappeared between 21:10 and 22:00 that evening.
Police have released details of the suspect’s phone number (+351 912 730 680) and the number which dialled him (+351 916 510 683), and said any information about these numbers could be “critical”.
Madeleine went missing shortly before her fourth birthday from an apartment in Praia da Luz on the evening of 3 May 2007 while her parents were with friends at a nearby tapas bar.
Her disappearance sparked a huge and costly police hunt across much of Europe – the most recent Metropolitan Police investigation, which began in 2011, has cost more than £11m.