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Early Tuesday, FBI agents raided his Zionsville, Indiana, home after obtaining a search warrant as part of the investigation. They also escorted Fogle, a longtime brand ambassador for the sandwich chain who once weighed 425 pounds and lost nearly 250 pounds with his own Subway diet.

WTHR reports Fogle has been detained outside his home though he has not been placed under arrest. Agents also removed electronics from his home.

Privately held Subway and Jared Fogle did not immediately respond to CNBC's request for comment. CNBC's call to Indiana State Police also was not immediately returned.

In a statement, the FBI said it "is conducting investigative activity in the area. We can not disclose the nature of the activity at this time."

Subway is the largest restaurant chain by locations with more than 44,000 restaurants in 110 countries.