Former President Lee Myung-bak was grilled by prosecutors on Wednesday over a string of corruption allegations including bribery, embezzlement and abuse of power.
"I stand here with a heavy heart", Lee said as he arrived at the prosecutors' office in Seoul, after a auto journey from his home in the capital covered live on television.
Lee made his way before hundreds of reporters waiting outside the prosecutors' office to ask him questions in the latest top-level political corruption scandal to rock the country.
"For those who trust and support me, and those who are suffering from the issues raised, I sincerely apologize".
"I have much to say as a former president but I promised myself to save my words".More news: White House Issues Order Prohibiting Broadcom's Bid for Qualcomm
Lee also said he hopes for the day's event to be the "last in history".
The former president, who was in office from 2008-2013, is suspected of being the real owner of DAS, which on paper belongs to his elder brother, Lee Sang-eun.
Former President Lee Myung-bak stands in the photo line at the Seoul Central District Prosecutors' Office early in the morning for questioning Wednesday.
The National Intelligence Service, the country's spy agency, is believed to have delivered secret operation fund to Lee's office at his behest through his closest aides, some of whom already admitted the allegation. The questioning resumed at 2 p.m. Lee has previously called the investigation "political revenge" orchestrated by the current liberal government of President Moon Jae-in, a statement that prompted fury from Moon.
On Tuesday, Lee and his legal team put their heads together at his home in Nonhyeon-dong, Seoul to prepare for interrogation.More news: McConnell says he hopes Senate Democrats do not block Pompeo or Haspel
Prosecutors will also question him on whether he owns DAS.
Lee, accompanied by his defence lawyer, was received by a senior prosecutor and given tea before being ushered into room 1001, where Park underwent a marathon 21.5 hour questioning previous year.
Almost all former presidents in South Korea, their family members and key aides have been embroiled in scandals either just before they ended their terms or after they left office. Should he be arrested, he would be the second former president behind bars on corruption charges.
Park Geun-hye, Lee's successor, was ousted from office a year ago, standing trial on charges of bribery and abuse of power. She is now standing trial, with prosecutors requesting 30 years in jail.
He was to be interrogated by three prosecutors and Yonhap news agency said the questioning - which will be videotaped - was expected to last around 20 hours, with prosecutors having prepared around 120 pages of questions relating to some 20 alleged offenses. "They should do their best to get rid of the wrongdoings of the past", the man in his 40s said. He is expected to deny charges against him.More news: France's Macron pledges €700m to solar energy push
Kim, who served as senior secretary for administrative affairs to Lee, was arrested on January 17 over allegation of receiving more than 400 million won from the National Intelligence Service between 2008 and 2012 at Lee's order.