BERLIN (AP) — 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 a far-right group called the National Socialist Underground.

Prosecutors accuse the group's three members of killing eight Turks, a Greek and a German policewoman between 2000 and 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 move increases the pressure on BfV chief Hans-Georg Maassen to explain what his agency knew about the NSU and why the cellphone and SIM cards — containing potentially important information — remained undiscovered for more than four years.

The previous head of the BfV resigned in 2012 following questions about authorities' failure to apprehend the group, whose existence only came to light in November 2011 when its two other members — Uwe Mundlos and Uwe Boehnhardt — died following a botched heist.

German media recently reported that two of the far-right group's members worked for another BfV informant, codenamed 'Primus,' while on the run. The agency said at the time it hadn't been able to confirm a link between the informant and the suspects.