Prosecutors on Saturday questioned a former Supreme Court justice suspected of having helped clear Lee Jae-myung, now the ruling Democratic Party's presidential nominee, of an election law violation charge last year in return for a post-retirement job.

The Seoul Central District Prosecutors' Office brought in Kwon Soon-il, sources said, as investigators are accelerating a probe into a high-profile urban development corruption scandal involving an asset management firm, called Hwacheon Daeyu Asset Management.

Two months before his retirement in September last year, Kwon is known to have presented a not guilty opinion in Supreme Court deliberations on the election law violation charge against Lee, then governor of Gyeonggi Province.

After retirement, Kwon landed a high-paying job at the asset management firm involved in the scandal-ridden land development project in Seongnam in the province governed by Lee.

Kwon was later mentioned as part of the "5 billion-won club" ($4.2 million) consisting of those to whom dubious land developers offered or pledged to offer massive amounts of money.

Prosecutors were expected to question Kwon over why he presented a not guilty opinion during the top court's trial last year and other related allegations.

Earlier in the day, prosecutors also questioned Kwak Sang-do, formerly affiliated with the main opposition People Power Party.

Kwak has been facing a probe following revelations that his 32-year-old son received 5 billion won in severance pay after seven years of work at Hwacheon Daeyu. Investigators suspect the money could be a bribe to Kwak. (Yonhap)