I say that it HAD to be an inside job. Someone on the inside of Woltz's estate had to be bought off. It's the only way that the job could have been done. Someone close and trusted by Woltz. My bet would be that the stable guy, which I think is named Tony, was the guy who was bought off. It did not appear to me that Woltz was the kind of guy who was loved by his workers. Now I don't think that Tom would have been the one to approach Tony the stable guy. It would have had to be done through a third party, perhaps someone from the Molinari family, as to not tie the job directly to Johnny and The Corleones.