Ja, I know, I am only some random person on the internet, I’m aware of that, and understand your wish for a source on that.
I can explain the source, but it’s (unfortunately) not going to be a link to a US website listing this dismeanour and the fine amount/range. So, I’m afraid it won’t be fully satisfactory.
The source is a Swiss lawyer (he was supposedly the specialised one for such topics of the medium-sized law firm) . I assisted/stood by a close friend who was going through such an inheritance (all-swiss/non-US people, low 7-figure inheritance, high 6-figure US stocks).
The fine was the only risk, the lawyer stated, for non-declaration at the “correct time”.
The DTA stands highest from a law point of view, and does not lose its effect by not filing.
I’d like to add, because some have mentioned “just fill out the form, doesn’t look too difficult” - I do remember, that he did also mention that smallish accounts at US banks (a few $ 1000 to $20000/30000), which will be blocked, are often simply “written off”, because the procedure to file is often complex & involves expensive lawyers (the form itself may be not too complicated, but for example attaching a valuation of Swiss RE conforming to US IRS requirements and standards quickly costs thousands!)