The prosecutors’ office is to summon higher-ranking National Intelligence Service officials as the investigation into the spy agency’s alleged evidence forgery moves up the ladder.

According to reports, the prosecution is set to begin summoning NIS officials in the chain of command above the agent detained on Tuesday.

The detained agent, identified by the surname Kim, is suspected of having played a crucial part in obtaining forged evidence against former Seoul City official Yoo Woo-seong. Although Yoo was initially acquitted of spying for North Korea, the prosecution appealed the ruling using Chinese government documents, which were later revealed to be forgeries.

The NIS officials high on the prosecution’s priority list are said to include Kim’s immediate superior, who is suspected of having been deeply involved in Yoo’s case from the initial NIS investigation to the submission of the forged evidence.

The investigators are also said to be analyzing a number of crucial pieces of evidence secured from the spy agency and the Ministry of Foreign Affairs.

Materials under scrutiny include a report compiled by Kim containing details about the process of securing the Chinese immigration documents and the NIS’ records on its investigation into Yoo.

In addition, the investigators are analyzing the computer used by a consul stationed at the South Korean diplomatic mission in Shenyang, China. The consul, identified as Lee, is an NIS official involved in the process of obtaining Chinese documents.

Along with Kim, an NIS collaborator is currently in custody in connection with the case. The collaborator, a Korean-Chinese also named Kim, claims that he was ordered by the detained agent to obtain the documents in question, and that the NIS was aware that they were forgeries.

As the investigators delved deeper into the case, the main opposition Democratic Party intensified its attack on the highest levels of the NIS and the government.

“The reason the investigation is not progressing speedily is that Nam Jae-joon remains (in office),” DP floor leader Rep. Jun Byung-hun said. Nam, the current chief of the NIS, was appointed by President Park Geun-hye. Since last year, the opposition bloc has been calling for his dismissal, claiming that he was involved in various irregularities including the developments following the 2012 presidential election.

“The president’s connivance is the reason Nam can maintain his position, and … (the president is the one) blocking the truth about the case.”

By Choi He-suk (cheesuk@heraldcorp.com)