Who set up the meeting that never took place, between Michael and the Five Families to renegotiate peace after Don Corleone's death?

Tessio (also spelled 'Tesio') arranged the meeting between Michael and the Five Families after Don Corleone's death, but this was a trap to assassinate Michael. Tessio had secretly aligned with Emilio Barzini, the main rival. Michael discovered the betrayal and had Tessio killed before the meeting could take place, eliminating the last of his father's old captains who turned against him.