By Se Young Lee and Ju-min Park SEOUL (Reuters) - Samsung Group [SAGR.UL] chief Jay Y. Lee was questioned by South Korea's special prosecutor for more than 15 hours as part of an investigation into a graft scandal that threatens to topple President Park Geun-hye. The 48-year-old executive, the third-generation leader of the country's top conglomerate, made no comment as he was shown on live television leaving the prosecutor's office in southern Seoul in a black car early on Tuesday. Lee, who has denied allegations of bribery through Samsung Group spokespeople, had arrived at the office on Monday morning.