This is the 18000th most frequent Korean word.
수사보고서였다
"Investigation report."
In this sentence, '수사보고서였다' refers to a specific investigation report used in solving a case.
그것은 범인을 찾는데 도움을 주었던 수사보고서였다.
That was the investigation report that helped to find the criminal.
Here, '수사보고서였다' denotes the subject matter of discussion in a prior session.
지난 회의에서 언급된 내용이 수사보고서였다.
The content mentioned in the last meeting was the investigation report.
In this instance, '수사보고서였다' describes the significant nature of a document as viewed by a perspective.