Lee Jong-sup (wearing suit and tie), the Korean ambassador to Australia, leaves a terminal at Incheon Airport after returning from Australia, Thursday. (Yonhap)

South Korea's anti-corruption investigation agency on Friday announced its decision not to summon Ambassador to Australia Lee Jong-sup for interrogation for a while. The decision comes amid escalating attention on the investigation into Lee, particularly regarding allegations of his unjust meddling in a military inquiry into the death of a young Marine on duty.

The Corruption Investigation Office for High-ranking Officials said "the investigation team of the CIO is in the position that it will be difficult for the time being to summon and interrogate the people involved" in a statement.

The CIO said "the digital forensics and data analysis on seized items surrounding the case have not been completed and the investigation of witnesses has not been sufficiently conducted."

The CIO also said that "the investigation team plans to do its best in the investigation under the given circumstances, review and evaluate the progress of the investigation, consult with the defense attorney and notify the people involved in the case of the date and time of the summons."

Lee returned home Thursday amid criticism of his appointment, as he was the subject of a CIO investigation into the death of the young Marine last summer.

Lee's return came 11 days after he left to assume the position of Korea's ambassador in Canberra. The main opposition Democratic Party of Korea and other critics have charged that Lee's appointment amounts to helping a suspect flee overseas and demanded President Yoon Suk Yeol withdraw the appointment and bring Lee home to face the investigation.

Lee is accused of downplaying the Marine's death and interfering in the investigation into the incident. Cpl. Chae Su-geun lost his life in July last year after being swept away by a strong current during a search and rescue mission for victims of a heavy downpour.

After arriving at Incheon Airport on Thursday, Lee told reporters that he came back to attend a meeting of diplomatic mission chiefs related to defense industry cooperation and that he wanted the CIO to question him while he is in Seoul.

"I hope that schedule coordination with the CIO will go well so that I can undergo questioning while staying here," Lee told reporters.

Lee submitted a petition to the CIO on Tuesday to demand it set a date for his questioning, according to the anti-corruption agency.

With Lee pressuring the CIO upon his return and insisting on an active investigation, the CIO finds itself in an awkward situation. Lee, who had been banned from leaving the country, was appointed as an Australian ambassador and went abroad, therefore, the CIO urged Lee to return home several times, saying he should return for investigation. But the CIO cannot now immediately conduct its investigation into Lee.

Since the investigation is still in the early stages, even if Lee is questioned at this point there is a high possibility it would be an ineffective investigation.

Lee's attorney, Kim Jae-hoon, told reporters Friday, "We are embarrassed by the position of the CIO that it is difficult to investigate Lee for the time being.

"Previously, the CIO extended Lee's ban on leaving the country several times and expressed opposition to lifting the ban on leaving the country. So it is difficult to understand that the CIO is not prepared for an investigation to summon Lee."

The ruling People Power Party has urged the CIO to quickly conduct its investigation into Lee.

Now that Lee has returned, "the CIO must quickly investigate and conclude the investigation," interim leader of the People Power Party Han Dong-hoon said Thursday. "If it is not ready yet, Lee’s investigation is something that the CIO and the Democratic Party of Korea used politically ahead of the general election."

Lee's whereabouts have become a focal point of debate among the two major parties as the April general election to elect the National Assembly approaches.

The ruling party has denounced the Democratic Party of Korea multiple times, claiming the main opposition “is using Lee’s case to win votes.”

In response, the Democratic Party has shifted its focus from Lee’s return to Yoon’s dismissal of the ambassador. Floor Leader Rep. Hong Ihk-pyo and Rep. Park Ju-min held banners urging Yoon’s dismissal of “criminal suspect Lee Jong-sup” during a protest they staged against Lee at the airport on Thursday.