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.
"As a former president, I have many things I want to say, but I promised to myself that I must save my breath", he said.
Lee has denied any wrongdoing, calling the investigation into the bribery allegations politically motivated by prosecutors under the incumbent liberal administration. They are Kang Hoon, 64, a judge-turned-lawyer who also served as Lee's former presidential legal assistant, Pi Young-hyun, 48, and Kim Byung-cheol, 43.
"I'm very sorry for causing concern to the people", he told reporters, adding that South Koreans' livelihoods were "difficult" and the security situation on the Korean peninsula was "dire".More news: Philippines Duterte to issue notice of withdrawal from ICC
The former president was in office from 2008 to 2013 before his successor, Park, won her presidency. Lee has denied all charges levied on him.
Samsung reportedly paid six billion won ($5.6 million) in legal fees to a United States law firm on the former president's behalf.
Lee is also suspected of dodging taxes through accounting fraud in the DAS and by owning real estate assets and bank deposits under borrowed names.
Lee is expected to return home after the questioning, but prosecutors are thought likely to then ask a court for an arrest warrant. The prosecution temporarily shut down most of its entrances and only allowed entry for media personnel whose IDs had been cleared in advance.More news: Teacher 'Accidentally' Fires Gun In Class
Former South Korean President Lee Myung-bak visited on Wednesday the prosecution headquarters for an interrogation over numerous corruption allegations, the Yonhap news agency reported on Wednesday.
Before he entered the prosecutors' office, Lee announced to reporters that he is in a "position of awful discomfort today".
Lee is the fourth former president to appear for questioning as a suspect after former President Roh Tae-woo did so in 1995, Roh Moo-hyun in 2009 and Park Geun-hye past year. "All I wish for is that this will be the last time in history" that a former president is being investigated.
Lee also referred to the 2009 suicide of ex-liberal President Roh Moo-hyun, who leapt to his death amid a high-profile corruption investigation of his family when Lee was president.More news: New Splinter Cell Game Spotted on Amazon Ahead of Announcement
Two other ex-presidents, Chun Dioo-hwan and Roh Tae-woo, were tried and convicted in the 1990s for bribery, treason and other charges for their involvement in a 1979 military coup and each spent two years in prison.
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