Who killed Fredo Corleone?

Can you clarify for me, who was responsible for the tragic death of Fredo Corleone? Was it an act of revenge from a rival crime family, a personal dispute gone wrong, or perhaps an internal conflict within the Corleone family itself? The question lingers, as the circumstances surrounding Fredo's death remain shrouded in mystery. Could you shed some light on this matter, and provide any insights or theories that might help unravel this dark chapter in the history of the Corleone crime family?