Uchiha Shisui was said to be the most skilled of the Uchiha clan. Several years previously he apparently committed suicide by drowning in the Nakano River. He and Uchiha Itachi were close comrades.
Both Itachi and Shisui were to meet at an Uchiha clan meeting, but both men did not arrive. When the Konoha Police Corps found Shisui's body, they began to investigate his death. A suicide note was found with Shisui's body, though it was his handwriting, anyone utilizing the Sharingan could have copied and mimicked his writing style. In the note Shisui said he was tired of his Uchiha duties and that the clan had no future. The police found it hard to believe a highly skilled ninja would suddenly give everything up and kill himself, they suspected that Itachi may have been involved. When Itachi was confronted by the police about his whereabouts, he grew angry at their accusations and their doubts so he began to attack the police. Itachi's father reprimanded his son and the matter was considered dropped.
Itachi wiped out the other members of his clan and told Sasuke he had killed his best friend Shisui to test his capacity and gain the Mangekyou Sharingan. To get this new Sharingan variation, he claimed one had to kill their best friend.