That's a good question. I think it was a backup team from Roth. But there's also some insinuation by Michael that it was done by people from the inside, who were very scared. But unfortunately this is never cleared up.
I think it was Fredo
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
