A key witness to prosecutors, who indicted a former prime minister, Han Myung-sook, for her allegedly receiving of a $50,000 bribery while she was in office, turned out to be their nightmare as he retracted his earlier statements and even claimed he felt pressed during questioning by prosecutors.

Kwak Young-wook, former CEO of Korea Express, a logistics company, is a key witness to prosecutors.

Based on Kwak's testimony, the prosecution team established the case that Han received $50,000 in two separate envelopes at the prime minister's residence on Dec. 20, 2006 from Kwak, who asked her to use her influence to get him appointed as the CEO of Korea Coal Corp, a state-affiliated company.

During a trial on Thursday, however, Kwak denied handing the money directly over to Han. "I placed the money on a chair (in the room). I am not sure whether Han saw the money or picked it up," JoongAng Ilbo cited him as saying.

Kwak earlier reportedly told prosecutors that the money was given directly to Han.

When Kwak was asked by the defendant's lawyer, Kwak said, "what I said at the court (was true)," while denying the verity of his earlier statement to prosecutors, according to the article.

Kwak's retracting is greatly undermining the ongoing investigation, which was criticized by Han's supporters as politically motivated.

Upon hearing Kwak's statement, "the prosecutor in charge of the case looked visibly shaken," JoongAng said.

The prosecutors' most valuable witness also embarrassed them with his other statements as well. When asked, "Why did you try to give the money to Han by going to the prime minister's residence," he answered: "That's because it was hard to meet her."

The remark undermined prosecutors' claim that Kwak maintained "a very close" relationship with Han.

During the court testimony, Kwak also openly said, "The prosecutors used a 'high-pressure' interrogation."

"The prosecutor was too scary that I even felt I would rather die," adding sometimes it was almost 3 a.m. when the returned to the jail where he is now serving his terms.

When he was asked about his retracting, Kwak said, "I was not thinking clearly."

The prosecution team previously all along said, "Kwak's statement formed the key basis for the investigation."

Now, observers say the prosecutors will be hard pressed to substantiate its charge against Han, not to mention to deal with its compromised reputation.

Supporters of Han claimed that the ongoing probe is a political maneuver aimed at damaging the political career of Han, one of the most prominent candidates from the opposition camp for the Seoul mayoral election in June.