The raid comes as part of a widening investigation into the alleged illegal provision of funds by the NIS to the presidential office Cheong Wa Dae during the two previous conservative governments.

Prosecutors believe that Kim Joo-seong, a longtime close associate of the elder Lee and a former NIS senior official in charge of the agency's planning and coordination unit, may have played a key role in the alleged secret funneling of the funds to the former president and his family.

Kim, who handled the NIS budget, was appointed to the post in 2008 even though he had no prior experience in the intelligence field. He had worked for over three decades at a retail company, for which the elder Lee once served as CEO.

Prosecutors said they are not excluding the possibility that the NIS fund was used by Lee for his political activities.

The investigation has been closing in on former President Lee, as a number of his close aides have been questioned for their involvement in the alleged delivery of illegal funds to the presidential office during his 2009-2013 term.

Prosecutors have also undertaken a cross-interrogation of a former assistant to then first lady Kim Yoon-ok and Kim Hee-joong, then the presidential secretary for private affairs, to verify Kim's confession that he delivered $100,000 from the NIS fund to her through the assistant. The ex-assistant has reportedly admitted to part of the claim.

The former president denied any wrongdoing and spoke out in a recent press conference, claiming that the investigation is political retaliation by the liberal Moon Jae-in government.

Park Geun-hye, an ousted and jailed former president, was indicted on additional bribery and embezzlement charges early this month in connection with the NIS fund scandal. (Yonhap)