As Vito immediately knew and Carlo confirmed later, it was Barzini all along. smile
Plus what Turnbull said about the "Bonnie and Clyde" shout out.
And given that Vito had had inexplicable luck in escaping two previous assassination attempts I suppose the people on the other side wanted to make sure there were no mistakes with Sonny.

"Forty seven shots! Forty seven shots in his Sicilian hide and he's still alive!"

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.