The Paris prosecutor's office said Claude Gueant is under questioning today along with the former national police chief, Michel Gaudin.
According to French media reports, the probe centres on envelopes full of 10,000 euros in cash delivered to Gueant every month from 2002-2004, when Sarkozy was interior minister and Gueant was his right-hand man. Gueant went on to serve as presidential chief of staff.
Sarkozy has not been publicly targeted in the case. But it could be one more potential problem for the former president if he seeks another term in 2017. Sarkozy has been investigated in a case involving illegal campaign financing and misuse of public funds.