Most likely a scam.  The threat to send officers to your niece's home for "interfering in an investigation" sounds like typical debt scammers who attempt to scare people into paying a bogus debt, a debt that's already been paid, or even a debt that is owed - but not to the caller.  The threat they made, by the way, is illegal.  Have your niece contact her local police department and make a report for harassment.  I'm sure the police will tell you it's a scam attempt. Your niece can also block their number - her phone may have this capability, or she may need to look at buying a call blocker.

I meant to add - it's likely that the number is being spoofed, so this David Hoffman probably has nothing whatsoever to do with the callers.  Scammers often spoof numbers - make it look like they're calling from someone else.