The resignation of Reinhard Boos as chief of domestic intelligence in the eastern state of Saxony brings to three the number of leading intelligence officers who have stepped amid revelations about the handling of the probe into the Nationalist Socialist Underground (NSU).

Based in the Saxon city of Zwickau, the NSU is thought to have been responsible for the murders of eight immigrants and a policewoman between 2000 and 2007, along with a series of robberies.

Chancellor Angela Merkel has already apologized to the families of the victims for the failures in the official investigations into the three-member cell.

Speaking to parliament, Saxony's Interior Minister Markus Ulbig said Boos had asked to be replaced because of "the glaring mistakes of individual employees."

Last week, the head of Germany's intelligence service, Heinz Fromm, offered to stand down. He was followed the chief of another state intelligence service in Thuringia, Thomas Sippel, who decided to take early retirement.