secret investigation; private enquiry; private inquiry; reconnaissance; scouting
警察が詐欺集団の内偵を進めていた。
The police were conducting a secret investigation of the fraud ring.
内偵の結果、不正の証拠が次々と見つかった。
As a result of the secret investigation, evidence of fraud was found one after another.
長期にわたる内偵が犯罪組織の壊滅に繋がった。
The long-term covert investigation led to the destruction of the criminal organization.