special investigation
Abbreviation of 特別捜査 (とくべつそうさ). Commonly used in police and legal contexts, often in compound terms like 特捜部 (special investigation unit).
特捜部が事件の捜査を始めた。
The special investigation unit has started investigating the case.
As a result of the special investigation, new evidence was found.
The special investigation squad launched an inquiry into the corruption case.
The special investigation team arrested the suspect.