The source was not allowed to speak publicly about an ongoing case and refused to say where the interviews with investigators were taking place today.
France's financial prosecutor opened a preliminary embezzlement and misappropriation of public funds probe of Fillon last week.
The investigation followed a report by Le Canard Enchaine newspaper that Fillon's wife was paid about 500,000 euros (USD 537,000) for work she did not perform.
Fillon says her job "was real."
It's not illegal for French lawmakers to hire their relatives as long as they are genuinely employed.
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