State prosecutors questioned several executives of telecom giant KT Corp. as part of their ongoing corruption probe into the company's chairman, prosecution officials said Monday.

Chairman Lee Suk-chae, who tendered his resignation Sunday, is under suspicion of inflicting huge losses to KT, the country's top fixed-line operator and No. 2 mobile carrier, through poor investments and dubious business deals, they said.

The executives, including the firm's official handling employee welfare only identified by his surname Shin, have been grilled, prosecutors said, without giving details on the date of the questioning.

The probe was launched after a local civic group filed charges of dereliction of duty against Lee with the Seoul Central District Prosecutors' Office in February.

The People's Solidarity for Participatory Democracy alleges that the chairman made bad investments in online education and subway network systems, causing the company hundreds of billions of won in losses.

Lee also allegedly played favorites with unauthorized subcontractors, the civic group claimed. The main complaint was that companies that were not included in the usual pool of KT's subcontractors obtained contracts, it added.

The civic group filed another charge against Lee in early October, accusing him of selling 39 office buildings of the company at below the market price, inflicting 86.9 billion won ($81.8 million) in losses to KT and its investors.

Furthermore, Lee is under suspicion of stashing secret funds by inflating the costs of salaries paid to the executives, sources at the prosecution office said.

State prosecutors raided the head office and affiliates of KT and residences of several executives, last week for the second time as part of the probe.

The prosecution office said it will soon summon Lee for further questioning after wrapping up the questioning of other executives.

The chairman, a close aide to former President Lee Myung-bak, took the helm of the mobile carrier in 2009. He also formerly served as a telecommunication minister in 1995. (Yonhap News)