Patricia Cahuzac was taken into custody on Wednesday and was to be questioned by anti-fraud police in the western Paris suburb of Nanterre, the source said.
Patricia Cahuzac, a dermatologist, and her husband, a former plastic surgeon, ran a flourishing hair transplant salon in Paris. Jerome Cahuzac also served as an adviser to several pharmaceutical companies.
The disgraced ex-minister, whose tasks in Socialist President Francois Hollande's cabinet included battling tax fraud, initially denied allegations by French investigative news website Mediapart that he had funds in a Swiss bank account.
He resigned on March 19 and has admitted to having the account for some two decades, in a scandal that dealt a major blow to Hollande's government.
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A retired tax inspector, Remy Garnier, told a parliamentary investigation panel that he has met with two private investigators hired by Patricia Cahuzac, probably in connection with proceedings in her ongoing divorce from Cahuzac. Both investigators were aware of the undeclared accounts, Garnier said.
A former official with Swiss bank Reyl et Cie who claimed he knew of 15 French politicians and "big names" with hidden bank accounts in the tiny Alpine country has meanwhile submitted the list to French investigators.