You searched for: “clew
clew, clue
clew (KLOO) (noun)
1. A ball of yarn or thread: In Greek Mythology, Theseus used a clew to find his way out of a labyrinth.
2. Primarily British, information that leads towards the solution of a situation or problem: The detective was able to use one final clew to solve the murder.
clue (KLOO) (noun)
Something that serves to guide or to direct someone in the solution of a problem or mystery: The spy provided a clue to the identity of the suspect.

In the mystery story, the inspector followed the clew of thread as a clue to finding the culprit.