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An AFP journalist saw detectives arriving at Netanyahu's official Jerusalem residence in two vehicles at around 9 am (0700 GMT).
Police issued no official confirmation that questioning was under way.
Investigators were at the same time to take a statement from his wife Sara at national fraud squad headquarters near Tel Aviv, media said.
A former Netanyahu aide and a top businessman are both in custody, linked to allegations of suspected fraud by the premier.
Nir Hefetz, a former media adviser to the Netanyahu family, and Shaul Elovitch, the controlling shareholder of telecoms group Bezeq, were among seven suspects arrested last week as fraud suspicions against Netanyahu snowballed.