Prosecutors have raided Samsung offices and questioned top executives, as well as Lee's family.
Samsung Group includes Samsung Electronics Co., a world leader in making computer memory chips, flat screen televisions and mobile phones.
Founded 70 years ago, the conglomerate also has interests in shipbuilding, insurance, apparel and other industries. One division is building in Dubai what it touts as the world's tallest skyscraper.
Lee, who took over leadership of the group two decades ago, is credited with making Samsung Electronics a top global brand by transforming its corporate culture to focus on quality.
Many South Koreans see the company as a source of pride.
Samsung, however, also has been a target of allegations by civic groups that it has used dubious financial transactions to ensure that control of the conglomerate passes from Lee to his son.
The independent counsel is examining those claims too.
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Associated Press writers Hyung-jin Kim and Jae-hyun Jeong contributed to this report.

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