This is the 6000th most frequent Russian word.
следственных
"Следственных" means "investigative" in English.
Here 'следственных' acts as an adjective describing the noun 'действий', referring to actions related to investigations.
Следственных действий велось много на прошлой неделе.
Many investigative actions took place last week.
Here 'следственных' describes 'органов', indicating authorities whose function relates to investigations.
Он изучил все материалы следственных органов.
He reviewed all the materials of the investigative authorities.
In this sentence, 'следственных' modifies 'документов', describing documents that pertain to investigations.