A detective at the centre of the Deutsche Bank spying affair says the international banking giant's effort to monitor its critics was more extensive than previously disclosed in that it involved a plan to target as many as 20 people, including a number of investors.

The detective's statements appear to contradict the bank's assertion that the spying affair was limited in scope. The affair came to light in May when Deutsche Bank said it had discovered efforts involving "questionable investigative or surveillance activities" directed by its corporate security department and outside contractors. It has so far disclosed four separate incidents involving its agents between 2001 and 2007 in which the bank used the detective, Bernd Bühner, and others to monitor people affiliated with the bank.