noun
controlled delivery; arm's-length surveillance
A law-enforcement tactic where authorities allow a suspect to continue illegal activity under surveillance in order to gather evidence or identify accomplices.
警察は泳がせ捜査で密売組織の全容を解明した。
The police uncovered the full scope of the trafficking ring through a controlled delivery operation.
おとり捜査 (sting operation) involves undercover officers actively luring suspects into committing a crime, while 泳がせ捜査 is more passive, letting existing criminal activity continue under watch.
Compound of 泳がせる (to let someone swim, i.e., to let someone act freely) and 捜査 (investigation). The term is a calque from English 'controlled delivery' or similar law-enforcement concepts.