The National Intelligence Agency said it determined Jang Song Thaek was removed from power after two of his confidants were executed in public, Yonhap reported.
"This is an indication that Jang has been dismissed, although it is not known why he fell out of favor," an NIS official told the South Korean news agency Yonhap.
Jang is the husband of Kim Kyong Hui, aunt of the North Korean leader. He has been considered the most influential figure in North Korea after Kim, and played a role in Kim's securing power after the death of his father Kim Jong Il in 2011.
Notable deaths of 2014 [PHOTOS]