I took care of my dad for the last 3 years. He died in November. I filed his taxes (separately; they have not filed jointly since approximately 2005) this year and included that he is deceased. He owed $150ish dollars so I included a letter saying he had no assets when he died; no property, savings, life insurance, etc. All his income amounted to was social security and a $100ish each month from a retirement account he had from a job 35ish years ago. I got a letter from the IRS last week saying he still owes that amount. Is my mother responsible for his taxes? Her only income is social security.