A congressional committee ordered the probe into how Fox managed to pay for what appear to be extensive renovations at his upscale ranch, the BBC reported Tuesday.
Fox, a former Coca Cola executive who reportedly was financially strapped while serving as a congressman, denies any wrongdoing, the British network said.
"He who owes nothing has nothing to fear," he said in a statement. "Ethics, transparency and full accounting have been the permanent norm throughout my entire life."
Fox reportedly has provided his tax returns from 2001 to 2006.
The probe into Fox's finances came after the magazine Quien ran an article and photos highlighting the substantial renovations at the family's home in the central state of Guanajuato. Earlier photos seemed to show a more modest residence.
Fox's wife, Martha Sahagun, and her sons are ensnared in a long-running scandal over government contracts, the BBC said. They deny an improprieties, as well.