The state anti-corruption agency summoned a key suspect in its investigation into alleged state intervention in the case of a Marine's death during a rain search and rescue operation last year.

The Corruption Investigation Office for High-ranking Officials(CIO) on Friday summoned Yoo Jae-eun, the defense ministry's general counsel, for questioning.

Prior to interrogation, Yoo said she planned to faithfully cooperate in the investigation, while declining to comment when asked about her phone conversation with Lee Si-won, presidential secretary for public office discipline.

Yoo, who faces power abuse and obstruction of exercise of one's right charges, is suspected of playing a role in retrieving from the police an initial report from a Marine probe into the death, and reducing the number of key suspects in the case.

This is the CIO's first official interrogation of a suspect in the case, after the agency conducted an informal questioning of former defense minister Lee Jong-sup last month.