泳がせ捜査

およがせそうさ【泳がせ捜査】

a criminal investigation [surveillance] where the suspect is given enough slack to lead investigators to critical evidence.

●警察の泳がせ捜査が効を奏して麻薬密輸グループが逮捕された. The police's arm's-length surveillance led to the successful apprehension of the drug-smuggling syndicate.

●泳がせ捜査をする maintain surveillance at arm's length [a distance].