The ruling Saenuri Party's Rep. Cho Hyun-yong was questioned by prosecutors on Wednesday in connection with an investigation into corruption in the railroad industry.
The prosecution is said to be planning to file for a preliminary arrest warrant against Cho on charges of bribery and violation of the Political Funds Act.
Cho is suspected of receiving large bribes from railroad track component maker Sampyo E&C in return for using his position to help the company win contracts for a number of railway projects.
"(I) will respond to the prosecution's questioning truthfully," Cho said before entering the Seoul Central District Prosecutors' Office.
He denied the allegations saying only that he would answer the investigators' questions.
Cho, a former Land Ministry official, has had a long career in rail-related positions including that of the Korea Rail Network Authority chief between August 2008 and August 2011. After entering the National Assembly in 2012, Cho served on the parliamentary Land, Infrastructure, and Transport Committee.
In 2011 and 2012, a newly track-laying system developed by Sampyo E&C was used in two separate rail projects. The company's products were shown to have developed faults including cracks in June 2013. However, the same product was selected for the high-speed rail line for the Jeolla provinces, raising suspicions that someone had influenced the decision.
While Cho denies the allegations, the prosecution is said to have secured testimony supporting the charges from Cho's associates.
According to reports, Cho's driver and his high school friend implicated in the case told the investigators that they relayed cash from Sampyo E&C to the lawmaker.
The company's CEO, identified by the surname Lee, and other officials questioned in the case are also said to have provided similar accounts.
Sampyo E&C officials, however, reportedly withheld details about the arrangements with Cho, while some denied that the money was provided in return for favors.
The prosecution is also said to have found evidence of suspicious flows of cash by analysing bank accounts held by Cho and other involved parties. The investigators suspect that Cho continued to receive bribes even after entering the parliament while serving as a member of the infrastructure committee.
So far, former Saenuri Party deputy spokesman Kwon Young-mo, a Board of Audit and Inspection official and Korea Rail Network Authority officials have been taken into custody in connection with the case.