Adam Werritty, Fox's former flatmate and best man at his wedding, was questioned by civil servants a day after Prime Minister David Cameron said the minister had made "serious mistakes" in his ties to the younger man.
Initial findings showed Fox, 50, had met the 34-year-old Werritty on 18 overseas visits since taking office in May 2010, and Werritty had visited him 22 times at the Ministry of Defence (MoD).
A British government source told AFP on condition of anonymity that civil servants had begun interviewing Werritty on Tuesday, but refused to give any further details about the interview or where it was taking place.
Fox has been under pressure for days after claims that Werritty posed as his advisor, brokered meetings with businessmen and accompanied him on ministerial trips including to Sri Lanka and Dubai, despite having no security clearance.
The defence minister addressed parliament on Monday, saying that the 18 trips where he met Werritty "in the margins" included family