Then-presidential candidate Moon Jae-in poses with his daughter Moon Da-hye and his grandson during a campaign rally in central Seoul, May 8, 2017. After winning the May 9 presidential election, Moon served as president from 2017 to 2022. Prosecutors are currently investigating allegations of preferential hiring involving his former son-in-law. Korea Times file

Prosecutors are intensifying their investigation into bribery allegations involving former President Moon Jae-in, focusing on claims that his former son-in-law was appointed as a local airline executive in exchange for favors granted to the firm's founder, officials said Sunday.

As part of the intensified investigation, prosecutors raided the residence of Moon's only daughter, Moon Da-hye, to assess the extent of the financial support that the ex-president and his wife may have provided to their daughter's family during his presidency.

The raid took place on Friday, and the warrant reportedly named the former president as a suspect in the bribery investigation.

This marks the first time that Moon Da-hye has been directly involved in the ongoing probe, which centers on her former husband, surnamed Seo, who was appointed as an executive of Thai Eastar Jet in 2018 during Moon's presidency.

Seo, who divorced Moon Da-hye in 2021, has been questioned as a witness three times earlier this year.

The investigation focuses on suspicions that Seo's appointment at the low-cost carrier, founded by former lawmaker Lee Sang-jik, was in exchange for favors granted to Lee. Seo was hired shortly after Lee became head of the Korea SMEs and Startups Agency in March 2018, during Moon's presidency. Given Seo's lack of experience in the airline industry, his hiring raised considerable skepticism at the time.

The Jeonju District Prosecutors Office, which is in charge of the case, has been accelerating the investigation in recent weeks, summoning Moon's former aides for questioning.

On Saturday, Rep. Cho Kuk, who served as a senior presidential secretary for civil affairs under Moon and currently leads the minor opposition Rebuilding Korea Party, was questioned for three hours. Cho denied the allegations, claiming that he does not know Lee or Seo.

Last Tuesday, prosecutors summoned Im Jong-seok, a former chief of staff for Moon, to inquire about his potential involvement in Seo's appointment. Im also denied the allegations.

The main opposition Democratic Party of Korea (DPK) criticized the extensive investigation, including the raid on Moon's daughter's home, labeling it as a "political vendetta" targeting the former administration.

"It would be a major mistake for the Yoon Suk Yeol administration to believe that humiliating a former president and using investigations for political revenge will distract the public from its policy failures," said DPK spokesperson Rep. Hwang Jeong-ah during a briefing at the National Assembly on Sunday.