BRAUNSCHWEIG, Germany (Reuters) – German and British investigators looking into the disappearance of British toddler Madeleine McCann 13 years ago are relying on hundreds of tips from the public to firm up evidence against Christian B., a suspected rapist they believe murdered her.

The 43-year-old German with a string of prior convictions was living in the Algarve region of Portugal when 3-year-old Madeleine vanished from her bedroom, sparking one of Europe’s largest-ever missing person investigations.

His emergence as a suspect in recent weeks has brought a the case back to the front pages of British newspapers.

Cellular phone data places the suspect near the holiday home where Madeleine was sleeping when she disappeared while her parents were at a restaurant in the holiday resort.

But more evidence is needed to be certain of keeping him in the prison in Kiel where he is currently serving a sentence for drug dealing, Hans Christian Wolters, a prosecutor in the central German city of Braunschweig, told Reuters on Friday.

Police broadcast a televised appeal for information last week in the hope that members of the public might turn up evidence needed to bring charges. Since then police in Britain and Germany have received “hundreds” of tips, Wolters said.

“Our investigation has turned up some evidence that was reason enough for us to go to the public, but it is an open secret that our suspicions are not firm enough to issue an arrest warrant,” he said.

Under German law, police have not released the suspect’s surname and media in Germany are not permitted to report it, although it has appeared in some British media. Calls to lawyers representing the suspect were not answered.

Braunschweig prosecutors are leading the investigation because the city was the last place the suspect had a fixed abode in Germany.

Wolters said the investigation would be more difficult if the suspect is released. He has already served two-thirds of his sentence, meaning a court could free him any day now.

Prosecutors have prepared another warrant for his arrest in a separate case over the 2005 rape of a 72-year-old American woman in Algarve, but the suspect is challenging the validity of that warrant at an EU court.

Wolters declined to comment on a report in Der Spiegel magazine on Friday that police had inadvertently warned the suspect they were investigating him over the McCann case as far back as 2013.

“I can’t say if those investigations were optimal or if there were any shortcomings,” he said.