Former South Korean President Lee Myung-bak was questioned Wednesday over allegations of taking 11 billion won ($10.3 million) in bribes from corporations and institutions, including Samsung, Hyundai Motor, and the country's spy agency.
"We will treat the former president with dignity but we will conduct a through and transparent probe", a senior prosecutor told journalists.
"I stand here with a heavy heart", Lee said as he arrived at the prosecutors' office in Seoul, after a vehicle journey from his home in the south of the capital that was shown on live television. "I am deeply sorry for causing concern to the public at this time when our economy is struggling and the security environment around the Korean Peninsula is very grave".
Former President Lee Myung-bak stands in the photo line at the Seoul Central District Prosecutors' Office early in the morning for questioning Wednesday.
South Korean presidents have a tendency to end up in prison - or meet untimely ends - after their time in power, usually once their political rivals have moved into the presidential Blue House.
His successor Park Geun-hye was ousted from office a year ago and charged over a massive influence-peddling scandal.
Now 87, he was initially sentenced to death, before the penalty was commuted to life in prison by the country's highest court in 1997.
Yoo Seong-min, head of the minor opposition Bareunmirae party, blamed the phenomenon on "the shortfalls of the winner-takes-all presidential government system" that creates opportunities for corruption. The inquiry will be videotaped as consented by the former president. Lee is expected to reiterate his claim that it belongs to his older brother Lee Sang-eun.More news: Grand Alliance, NDA poised for retaining their seats in Bihar by-polls
"This will boomerang" on Moon, he wrote on his Facebook account.
The prosecution is considering requesting an arrest warrant for Lee if he denies the allegations under questioning. They say the prosecutors have already obtained statements from Lee's aides that prove the allegations.
The former conservative leader, who was in office from 2009 to 2013, is the fifth president to be grilled by the prosecution, following Chun Doo-hwan, Roh Tae-woo, Roh Moo-hyun and Park Geun-hye.
In return for the legal fee offer, Samsung Electronics Chairman Lee Kun-hee is believed to have bought a presidential pardon in 2009 when the Samsung chairman, now in hospital, had got a suspended jail sentence for tax evasion.
When asked by a reporter if he accepted the allegations against him, Lee did not respond.
According to reports, Lee is also accused of accepting 2.2 billion won from a former CEO of a state-financed banking group for helping him assume the post.
Lee faces nearly 20 charges and the prosecution believe he took around 11 billion won unlawfully from a number of institutions and individuals including the country's National Intelligence Service and Samsung Group.
Lee is suspected of having Samsung Electronics shoulder lawsuit fees of about 6 billion won, or more than 5 million dollars, for an auto-parts company he effectively controlled.