The move came about two weeks after prosecutors demanded a 30-year prison term for Lee's conservative successor, Park Geun-hye, over a separate bribery scandal that led to weeks of massive anti-government protests. Park is in jail as she awaits a court verdict set for early next month.
"I stand here today feeling wretched," Lee said in televised remarks after arriving at a Seoul prosecutors' office. "I am very sorry for causing worry for the people."
South Korean politicians accused of misdoing often apologize for causing trouble while still denying wrongdoing. 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.
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. Moon, who was Roh's chief of staff, previously called the Roh investigation by the Lee government politically motivated.
Prosecutors accuse Lee of taking about 1.75 billion won ($1.6 million) from the National Intelligence Service, the country's main spy agency, for personal purposes through his aides.
Prosecutors allege that Lee took other illicit money from business groups including Samsung and a former lawmaker. South Korean media said the illicitly obtained funds totaled 11 billion won ($10 million).
Lee is also accused of using an auto parts manufacturer called DAS as a channel to establish illicit slush funds amounting to 30 billion won ($28 million) and embezzling its official funds.
One of Lee's brothers is DAS's largest shareholder, but prosecutors suspect Lee is its de facto owner.
Nearly all former presidents in South Korea, their family members and key aides were embroiled in scandals either just before they ended their terms or after they left office.
South Korea's first president, Syngman Rhee, resigned and fled to Hawaii in 1960 amid protests over allegations of vote-rigging. Park's father, Park Chung-hee, who ruled South Korea for 18 years following a 1961 coup, was gunned down by his own spy chief during a 1979 drinking party.
Park's successors Chun Doo-hwan and Roh Tae-woo, both former army generals, spent time in jail after leaving office for bribery, treason and other charges. Presidents Kim Young-sam and Kim Dae-jung saw their terms tainted by corruption scandals involving their sons.
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