I think they can’t be just making a case on that evidence alone. The Feds don’t usually charge someone unless they think it’s solid, that’s why they have some insane conviction rate like 97%.
My guess is that the unnamed mistress has perhaps flipped under federal questioning and is going to be a witness for the prosecution. They may not be able to prove she helped plan the murder, but since she married him and benefited from the insurance fraud, that was probably enough to threaten her with a stiff federal sentence if she didn’t point the finger at the husband.
“My guess is that the unnamed mistress has perhaps flipped under federal questioning and is going to be a witness for the prosecution.”
My guess as well. I bet the young mistress is no longer quite so young and the wandering dentist started spending nights away from home.