Fillon plans to hold a news conference today at his campaign headquarters in Paris.
Prosecutors are trying to figure out whether the jobs that Fillon gave to family members as his parliamentary aides were genuine. The preliminary probe involves suspicions of embezzlement and misappropriation of public funds.
Fillon's popularity dropped following revelations by the Canard Enchaine newspaper last month. He has denied any wrongdoing.
Some conservative lawmakers want him to step down to improve the party's chances to win France's two-round April-May presidential election.
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