Investigators are seeking to establish whether former president Nicolas Sarkozy, with the help of his lawyer Thierry Herzog, attempted to pervert the course of justice.
They are pursuing a line of enquiry that Sarkozy and his lawyer tried to use their influence to secure an attractive job for one of the magistrates, in return for inside information about the progress of the so-called Bettencourt affair.
In fact the magistrate concerned, Gilbert Azibert, was not appointed to the job in question.
The investigators are also trying to find out whether Sarkozy was illegally tipped off that his phone was being tapped as part of investigations into another matter, the so-called Ghadaffi affair.
The offices of both Gilbert Azibert and Thierry Herzog were raided by police in early March. Computer equipment a mobile phone belonging to Herzog were seized.
Suspecting that his phone was bugged, Sarkozy used a special mobile phone, registered under the borrowed name of Paul Bismuth, for conversations with his lawyer.
Since his defeat in the 2012 presidential elections he has been the subject of numerous judicial enquiries although he has not been convicted of anything.
His supporters suggest that the accusations and enquiries are politically motivated and intended to obstruct any return to national life.