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 apologise 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.
Stephen Evans, South Korea correspondent: A parade of apologies

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 apologised, 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.
Lee Jae-yong
Samsung's heir apparent

    - Grandson of Samsung founder Lee Byung-chul, son of current chairman Lee Kun-hee.

    - Aged 48, he's spent his entire career in the company and is vice chairman of Samsung Electronics.

    - Last year was nominated to join the board of Samsung Electronics - an appointment confirmed on 27 October.

    - Widely expected to take overall control of Samsung once his 74-year-old father steps down.

    - Critics say his position on the board is due to his birth, not his business experience.

At the parliamentary hearing in December, Samsung admitted giving a total of 20.4bn won (£16m; $17.46m) to the two foundations, but denied seeking favours.

And Mr Lee also confirmed the firm gave a horse and money to help the equestrian career of Ms Choi's daughter, Chung Yoo-ra, something he said he now regretted.

Mr Lee is currently vice-chairman of Samsung Electronics. But since his father, Lee Kun-hee, suffered a heart attack in 2014, he is considered de facto boss of the entire Samsung Group conglomerate.

Politicians voted on 9 December to impeach President Park over the scandal - a decision South Korea's constitutional court has six months to uphold or overturn.

Until then she remains formally president but stripped of her powers, which are handed to the prime minister, a presidential appointee.

Ms Choi is on trial for charges including corruption and coercion.

Ms Park's position began to unravel in October last year when details of her friendship with Ms Choi began to emerge.

They included revelations that the president had allowed her old friend - who holds no government role - to edit political speeches.

Since then, hundreds of thousands of protestors have gathered every weekend in Seoul to demand Ms Park stands down.

Ms Park denies wrongdoing but has apologised for the way she managed her relationship with Ms Choi, who also denies committing criminal offences.