Samsung chief Lee Jae-yong is being 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.
Connoisseurs of the apology will study this case for years to come. There has now been a string of important people saying they are deeply sorry, even as they profess their innocence of wrong-doing.
On his way into the investigator's office, Jay Y. Lee said he was sorry for portraying a bad image. In the past, President Park said she was sorry - for being too trusting.
And her mentor, Choi Soon-sil, also apologized, saying she had "committed an unpardonable crime". What crime that was though remains unclear - since she also said she was innocent!
Incidentally, Mr Lee has a record of apologies. Four years ago, he took his son out of a school after it was revealed that the boy had a space there meant for the underprivileged (which the son of the acting-head of Samsung clearly is not - in the land where the son always rises, the lad may well end up as the head of the company himself).
There will be more apologies before the current saga is over.Read More