Can a collection agency charge interest on medical and/or utility bills?

Question Details:

I have been researching this for a while and can't get a clear answer. Some sites have said they can in all situations, some have said they can if the original medical or utility bill allows for interest accrual, and some have said they can charge unless the bill specifically says they can't. The account was assigned to a collection agency and is not a judgment. As far as I can tell so far both my medical and utility bills did not have a provision for interest accrual, only "late fees".

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