The son-in-law of wealthy property owner Hélène Pastor is also among those being questioned following her ambush murder in May in Nice.
The mother, who made a tearful appeal for her missing five-year-old daughter, has admitted the girl is dead after she and four others were arrested.
The two French citizens killed in attack on Nairobi shopping mall were a mother and daughter who were 'executed' in the parking lot.
Police enquiries centre on late husband of a mother who disappeared with her daughter in July and who had a mistress who also vanished in 2004.
A 52-year-old Frenchwoman has been caught sitting a crucial pre-university English examination in Paris in her 19-year-old daughter's place.