A man who volunteered for former Prime Minister Lee Wan-koo's by-election campaign in 2013 was interrogated over allegations his former boss took bribes from a businessman, prosecutors said Wednesday.

Lee stepped down as prime minister last month after the businessman's suicide note implied he took 30 million won ($28,000) in kickbacks when he was running for a seat in South Chungcheong Province.

Lee denies the allegations.

Prosecutors said they have called in the former volunteer, identified only by his surname Han, who had told local media that he saw Sung Wan-jong, the late head of Keangnam Enterprises, visit Lee's campaign office on April 4, 2013, the date written in Sung's suicide note.

Sung also told a local newspaper right before his suicide that he paid Lee on that date.

Prosecutors are focusing on whether Han witnessed the incident and if there are other witnesses.

Sung hanged himself on April 9, leaving behind a note listing eight names, including Lee and several of President Park Geun-hye's close aides, next to what appears to be sums of money. (Yonhap)