get to the root of the matter; go back to the source(/origins); go into (/have)the why and wherefore of it; investigate (/probe)into the origins of; search(/get down) to the bottom of; sift into a matter to the bottom; trace (back)to the beginning (/source/very roots)of; track down the origin
❍ 十年后，人们仍要求对那个神秘的事件～。Ten years later people still demanded to inquire into the root of the matter concerning that mysterious accident.
追本穷源zhuī běn qiónɡ yuán
追：追究；本：根本；穷：深入探究。指追究探索事物发生的根源。trace to it’s source, search to the bottom of, go to the fountain-head, get to the root of the matter, want to hear the details, go into the whys and where fores of sth.