Deceased Mother's ssn# being used what should I do?

My mother passed when I was 6 yrs old and I am now 23 i'm just curious about what's going on cause my dad called me and told me a tax collection agency called his house asking for my mom and they been separated since 1993 when she passed well some how she open a loan and never re payed it back in 2009 like my family is very secretive on things i really think it's them that has her ssn# like i was trying to claim her on my tax returns just to see if someone else is really using it you know...and guess what (IT SAID THIS SSN# HAS BEEN CLAIM ALREADY WOULD YOU LIKE TO DELETE THIS DEPENDENT?) :o i would like to seriously find out about this cause my family don't care about me and my brothers at all so i want to protect my mother's ssn# hell so i also tried a credit report and it has a security freeze on her ssn# someone is very smart so please help me out thanks!!!.