vacant home, owner deceased, who to contact

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Any advice on this situation?  I found a home that was gifted to an individual who also passed away 5 months later in mid 2015.  Nothing has been filed for probate so I don't know of any heirs to contact.   I've sent letters to the property address and they are returned.   I'm attempting another letter addressed to "current resident" instead of the owner's name.   The appraisal district does not show any other address or other names.   No other documents have been filed with the county in reference to this address.  What if there are not any heirs?  I'm trying to find out who to contact.  Any suggestions or advice is appreciated. 

Lisa Rankin