Prosecutors on Sunday again summoned key suspects of the high-profile land development scandal for continued questioning amid growing calls for an independent counsel investigation into the case.

Kim Man-bae, who is believed to be the founder and a major stakeholder in Hwacheon Daeyu, appeared at the prosecutor‘s office for the fourth round of questioning, which came just three days after a round of questioning on Thursday.

Another key figure, Nam Wook, the second-largest stakeholder of Hwacheon Daeyu, was also summoned by prosecutors, two days since his last questioning.

A series of questionings of key suspects are coming as the prosecution faces hurdles in the investigation.

The prosecution this week released Nam, who has been accused of taking the initiative in the urban development project as a private partner, without seeking an arrest warrant.

In mid-October, the court also rejected the prosecution’s request for an arrest warrant for key suspect Kim, citing insufficient justification for detaining Kim.

Kim has been accused of colluding with Yoo Dong-gyu and arranging a profit-sharing scheme that favored Hwacheon Daeyu, costing the city of Seongnam, Gyeonggi Province, more than 110 billion won ($92 million).

The prosecution on Thursday night indicted Yoo, who served as an acting president of Seongnam Development Corp., on charges of bribery related to the real estate development.

Although Yoo is the first and sole suspect under indictment among the main culprits of the scandal, prosecutors have encountered criticism for failing to prove Yoo’s breach of duty charges.

Prosecutors initially sought an arrest warrant for Yoo on charges of bribery in connection with the project as well as breach of trust, but they removed the breach of trust charges in a written arraignment.

The core of the investigation is to prove the allegations of breach of trust so as to figure out whether Yoo intentionally inflicted losses on Seongnam, designed the profit-sharing scheme and deleted a clause that required the city to reclaim excess earnings from private partners.

Since South Korean prosecutors failed to prove the key misconduct allegations on the part of Yoo, voices calling for an independent counsel investigation into a high-profile land development corruption scandal are growing.

As the investigation continues to hit snags after around a monthlong investigation, the need for special prosecution is reportedly being raised within the prosecutor’s office with self-deprecating comments.

Some prosecutors see a necessity to launch a special counsel probe to put it on the right track, local media outlets reported over the weekend.

The Chosun Ilbo on Saturday pointed out that there was also an indication that the leadership of the prosecution showed a passive attitude to investigating an alleged breach of duties.

An unnamed official at the prosecution, cited by the Dong-A Ilbo, said it was hard to understand why the charges of breach of trust were excluded from the written arraignment.

South Korea’s main opposition People Power Party has ramped up its push for a special counsel probe into the land development scandal, which could have ramifications for the upcoming presidential elections slated for next March.

The party on Friday called prosecutors to get to the bottom of the scandal through an investigation by an independent counsel in a written statement. On the same day, the leadership and members visited the Supreme Prosecutor’s Office in protest, calling for a thorough investigation to determine the mastermind behind the development project in Daejang-dong, the neighborhood at question within Seongnam.

Yoon Seok-youl, a leading presidential contender for the People Power Party, claimed that prosecutors have blocked an expansion of the investigation, saying it is unusual to remove charges stipulated in the issue warrant in the indictment process. The former prosecutor general also said a special counsel investigation should be launched “at the earliest possible time” before evidence is destroyed, in a public statement on the issue Saturday.

But the ruling Democratic Party of Korea on Friday argued the arraignment proved that prosecutors forcibly applied breach of charges against Yoo to link a certain person to the land development scandal.