Fillon himself has already been charged in the case, which has deeply damaged the conservative candidate's chances for the two-round election April 23 and May 7. He is suspected of embezzlement and other charges.
Penelope Fillon is expected to face investigating judges today in Paris who can hand her preliminary charges in the case.
Investigators recently expanded the probe on suspicions that the couple falsified documents after the investigation opened to prove that Penelope carried out parliamentary work for her husband.