A tip flagging Madeleine McCann suspect Christian Brueckner to German authorities seven years ago was allegedly ignored.
Police in Braunschweig, Germany, received a tip about about Brueckner in 2013 and sent it on to the Federal Office of Criminal Investigation, according to reports.
But local reports say the tip was ignored.
German magazine Spiegel reported: "One person did submit a tip about Brueckner but the resulting report from police in Braunschweig to the Federal Office of Criminal Investigation was apparently not acted upon, much to the consternation of the local investigators."
According to the Daily Mail, at the time police were constantly monitoring the 43-year-old suspect.
This comes as Portuguese police face fresh scrutiny over missed opportunities to focus on the local sex criminal in their investigation into the disappearance of Madeleine McCann.
They have previously been widely criticised for their handling of the case, missing chances to secure potentially key forensic evidence in the apartment room from where Madeleine was taken.
Madeleine, then three, was abducted from a room in the Ocean Club holiday complex in the area on May 3 2007.
Brueckner is known to have been living in Praia da Luz between 1995 and 2007.
His Portuguese mobile phone was in Praia da Luz when it received a half-hour phone call around an hour before Madeleine, three, was last seen.
Brueckner was known to Scotland Yard's investigation but did not become an official suspect until 2017 following information received after the 10-year anniversary appeal and subsequent inquiries, the force said.
The force's Operation Grange still considers the case a missing person inquiry because there is no "definitive evidence whether Madeleine is alive or dead".
It emerged this week that Brueckner was in jail after being convicted of raping a 72-year-old American woman in Praia da Luz two years before Madeleine went missing
He has also been convicted for sex offences against girls.