Karim placed the blue thread inside a clean envelope so he would not lose it.
He remembered that the school used blue cloth for the curtains in the main hall, but the thread looked thicker.
During lunch, he quietly examined bags, coats, and instrument covers in the practice rooms.
He was careful not to touch anyone's belongings without permission.
Near the back stairs, he noticed a damaged blue cover on a large cello case.
The cover belonged to Sami, a friendly student who had performed at the school for three years.
Sami laughed when Karim showed him the thread and said the material did look similar.
He explained that the cover had torn during Friday's rehearsal.
Karim asked whether Sami had visited the storage room.
Sami admitted that he had gone there to look for spare music stands, but he said the violin was still inside its case at that time.
He also remembered hearing a heavy object move in the corridor shortly before he left.
Karim wrote the details in a notebook because he knew memory could change quickly.
The blue thread was no longer proof of theft, but it had led him to the first useful witness.