Legal Heirship Issue

I am an American Citizen (NRI). When my Mother passed away in 1990, my Father came to live with me and eventually became a US Citizen. My Father and I both hold OCI cards. My father passed away intestate in 2013 and I am his only surviving Son and Heir. The Tahisildhar's office in Kerala is questioning whether I can obtain a Legal Heirship Certificate since my Father was an American Citizen and has refererred the question to the General Pleader's Office. The case is under review by the GP office for their opinion. My Father has property in India that he obtained through inheritance and through family acquisitions (purchases) when he was residing in India. He also has a bank account that has his and my Mother's pension money. I currently hold a Legal Heirship Certificate for my Mother listing my Father and I as her Heirs. I also have a List of Interested Persons naming me as the sole Heir, and this document was registered in the Maryland Circuit Court upon his passing. The Bank in India is asking for a Legal Heirship certificate to release his pension money into my NRO Account. Kindly provide your valuable advise.