I don't know if it was before or after apalachin hoover had his fbi cronies illegally bug a bunch of social clubs/known mob hangouts to catch up on information because with hoover's insistence there was no national syndicate the fbi was woefully behind. I think it happened after bobby kennedy ripped hoover apart during a senate hearing.

After that, the fbi illegally bugged all the clubs but couldn't understand half of what was going on because they didn't understand the lingo, and I think it was when joe valachi ratted that he told them that la cosa nostra means this thing of ours.

I'm pretty sure I got all of this from Selwyn Raab's 5 families book