究其根源

to trace something to its source (idiom)

root / basis / classifier for long slender objects, e.g. cigarettes, guitar strings / CL:條|条[tiao2] / radical (chemistry)

his / her / its / their / that / such / it (refers to sth preceding it)

root / source / origin

origin / root (cause)

after all / to investigate / to study carefully / Taiwan pr. [jiu4]