Frankie was still the only high level mob boss who had been willing to rat Michael out. By keeping Frankie sequestered was the only leverage that the government had against Michael.
Eventually, the Fed would have tried to understand why Frankie had reneged on his testimony and would have attempted to overcome the obstacles and have Frankie testify against Michael in the future. Not being a court of law, double-jeopardy was not an issue.
This is why even though Frankie recanted before congress, Michael still wanted him dead to prevent him from testifying at some future time.
Also Frankie believed that it was Michael who tried to have him killed, remember when the Rosato Brother came up behind him and said, “Michael Corleone Says Hello”.
The question is, did they intentionally stop short of killing Pentageli, knowing then he would turn against Michael.
Maybe. I think it more likely that it was simply the necessary punishment for breaking omerta.
Delta - Respectfully, I don’t agree with the explanation of the Frankie Pentangeli issue, at least as far as the Feds keeping him sequestered. First, Frankie would have been sworn under oath before Congress, so even though “jeopardy” wasn’t an issue, even the worst defense attorney could entirely discredit his testimon in a subsequent trial. Further, even if he wasn’t sworn, Frankie would’ve been a totally useless witness later on. (I know this because I used to be a prosecutor). Once Frankie screwed up the Congressional testimony, he was utterly useless as any kind of witness later on, in any proceeding. In reality, had he done that he would’ve spent the rest of days in solitary.
I can understand why Michael wanted him dead, and basically struck the same bargain with him - die and we will take care of your family - as he did in the novel with the death row inmate who confessed to shooting McCluskey the police captain, thus getting Michael off the hook.
But this leads to another thing that didn’t make sense - after the Congressional hearing debacle the Feds would’ve never let Frankie Pentangeli have a private meeting with Tom Hagen (where Tom proposed the “deal” that led to Frankie committing suicide). Think about it, Tom was not Frankie’s attorney, but rather the attorney for the target of the investigation. No way in hell that happens.