Germany's interior ministry has ordered an independent investigation into the failings of the country's domestic intelligence agency in a high-profile neo-Nazi case.

The BfV agency recently acknowledged finding in its archives a cellphone and five SIM cards belonging to an informant codenamed 'Corelli.'

German lawmakers are investigating whether Corelli had links to the far-right National Socialist Underground, accused by prosecutors of killing nine men and a policewoman in 2000-2007.

The only surviving alleged member, Beate Zschaepe, has been on trial since May 2013.

The interior ministry said Friday that a retired senior civil servant will investigate the BfV's handling of evidence and submit a report at the end of the month.

The previous head of the BfV resigned in 2012 following questions about authorities' failure to apprehend the group.