Rep. Lee Yong-joo showed up at the Seoul Southern District Prosecutors' Office around 3:50 p.m., after he was summoned by the prosecution on Tuesday, from source of the Yonhap News Agency.
The prosecutors are expected to quiz him over his involvement in and knowledge of the creation and disclosure of the false allegation that Moon helped his son get a job at a public agency in 2006, using his clout as a presidential secretary. Lee has denied any wrongdoing.
The People's Party used the disinformation in a smear campaign against Moon in the run-up to the presidential election early this year. It turned out to be concocted by a former campaigner for then presidential contender Ahn Cheol-soo of the minor party.
"I had no knowledge of the fabrication, and I will tell (the prosecution) everything I know in the way they are," Lee told reporters as he went into the building.
They said earlier that Lee is to be questioned as a witness in the case as of now.
The prosecution has arrested two party members -- Lee Joon-seo and Lee You-mi -- on charges that they created and relayed the disinformation to the party's leadership. Rep. Lee is the person who received the fixed evidence from Lee Joon-seo.