David Shimron, Netanyahu's cousin and his personal attorney, arrived for questioning following revelations that he represented the German firm involved in the USD 1.5 billion deal, raising the prospect of a conflict of interests.
Shimron told reporters after leaving the investigators' offices near Tel Aviv that he "must respect the investigation and the investigators," and that he therefore could not comment.
Police suspect Shimron was hired by the Germans because of his ties to Netanyahu so he could push the deal through.
Israeli police have declined to comment on any details of the investigation.
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Suspicions of impropriety were heightened when former Defense Minister Moshe Yaalon said he was sidelined on the purchase plans -- which went forward after he was replaced last year.
A spokesman for German Chancellor Angela Merkel also declined commenting on the investigation, saying it was an internal Israeli matter.
Israel has ordered six submarines from Germany in the past two decades, with the final one scheduled for delivery in 2018. Israel recently decided to purchase three new vessels to replace its older submarines.