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Samsung chief Lee Jae-yong has been questioned at the prosecutor's office in Seoul as a suspect in South Korea's biggest political corruption scandal.
The firm is accused of giving donations to several non-profit foundations operated by Choi Soon-sil, a confidante of President Park Geun-hye.
The donations were allegedly made in exchange for political support of a controversial merger.
The scandal has led to President Park being impeached last December.
"I deeply apologize to the people for failing to show a positive image because of this incident," Mr Lee told reporters upon arriving on Thursday morning.
Earlier this week two other Samsung executives were interviewed by the special prosecutors, but were treated as witnesses rather than suspects.
The claims against the company circle around a merger between the electronics giant's construction arm, Samsung C&T, and an affiliate firm, Cheil Industries.
Prosecutors allege that Samsung gave €2.8m euros ($3.1m; £2.5m) to a company co-owned by Ms Choi and her daughter, in return for Ms Park's support for the deal.
Lee Jae-yong, also known as Jay Y. Lee, has already given evidence to politicians over the scandal, but this is the first time he has been quizzed as a suspect by investigators.