1) Pile of evidence that someone is guilty.
2) Federal "expert witness" allowed and funded to testify it points to X person
3) Defense is broke
4) If defense not broke, judge finds reason why defense expert witness can't present their full argument
5) Prosecution offers plea deal or a gazillion years in jail
6) Person folds when they realize none of their defense is allowed or will be funded, or goes to trial and gets a gazillion years for daring to go to trial.
If you already have a pile of evidence for a crime and an expert witnesses you can keep piling on it's not much of a stretch to dump it on some random person, as we saw here, quite easy actually.