The probe is at this stage only a preliminary investigation, the first step in the judicial process. Fillon has said he will abandon his presidential bid if he is placed under formal criminal investigation.
If police find that the allegations stand up, prosecutors can seek a formal inquiry by an investigating magistrate, which would take months to reach a conclusion.
However, prosecutors could also leapfrog that step and go directly to trial, the source close to the case said.
Should Fillon drop his presidential bid, time is running out for the conservative Republicans party to choose another candidate.
The party has only about two weeks to organise a new primary as a March 22 deadline approaches for all candidates to officially register for the election, Anne Levade, who oversaw the party's primary in November, told Le Monde newspaper at the weekend.
A source close to the case said businessman Marc Ladreit de Lacharriere had also been questioned on Monday because his Fimalac <LBCP.PA> holding company owns the literary review La Revue des Deux Mondes, which Le Canard Enchaine said had paid Penelope Fillon another 100,000 euros for very little work.
A Fimalac spokeswoman declined to comment on the questioning of Ladreit de Lacharriere, and there was no immediate response to an attempt to reach him by email.
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