Death and credit card death.

My father recently died leaving enough money to pay off most of his debts,but not all.I contacted those credit card companies that there no funds to pay and informed them of my father's death and that there were no assets to pay these debts.I soon received a letter (from american express) asking to pay the overdue minumum payment of $20.00.sears has a collection agency claiming to be a law firm that called(did not answer) and sent a somewhat threatening letter(have not answered) asking who the personal representitive or attorney hired as such was.I had previously talked to an attorney but did not think his help was needed. There is no probate,no assets to speak of,no personel representitive.what assets my father had(house and car had been in a trust)were passed on to me at my father's death.there were a few savings bonds with his name and my mother's(deceased in 2000)which he never removed but simply cashed after the 1 year waiting period.these I sent to the bureau of debt with my father's and mother's death certificates.they may be returned to me in cash at some point months from now. At least I assume that to be the case.there is about $700 in back retirement pay which will be sent to me at some point.he was retired military and civil service.this a very slow process.the only information these a collection agency would have fromme is my father' address and my home phone.should I communicate with the collection agencies in any way? None of these credit debt(about $6500) are mine. What happens if I ignore them.