Supplementary investigation power lets prosecutors further investigate cases handed over by police. Removing it shrinks prosecutors' room for direct investigation and expands the police role.

The ruling party cites separating investigation from indictment to ease the concentration of prosecutorial power. Parts of the legal community and the opposition, wary of shoddy investigations and delays, demand safeguards.

The crux is what fills the gap after abolition. Analysts warn that unless stronger police investigative capacity, oversight and a revamped cooperation system move together, citizens could be harmed.

The government and ruling party say they will refine procedures through follow-up legislation and decrees, but critics counter that pushing ahead without adequate preparation makes on-the-ground confusion inevitable.

Because restructuring investigation directly affects citizens' redress of rights, foreign residents including Chinese in Korea should watch how the procedural changes may affect them.