Mandarich Law Group, LLP Debt Collection Complaint

Mandarich Law Group, LLP Debt collection department,

I do not know Taking/threatening an illegal action Sued w/o proper notification of suit California

I am just at my wits end. I am being threatened with a suit by some huge Law Firm called Mandarich Law Group , LLP for a debt I have no knowledge of. I am a victim of Identity theft and I have lived at my current resident for several years and I have never received a notice from this law firm until now. In addition, they ( Mandarich Law Group ) claim they are collecting for their client who is - - - whom I have never been contacted by, however they are a collection agency. The collection agency according to my credits says they are collecting for -? Is this legal can I in fact be sued for something I know nothing about? I was never contacted by - who is suppose to be the original creditor, nor have I been contacted by - , - who is suppose to be the collection agency collecting on behalf of the original creditor. I was only contacted by the law firm with a DEMAND letter dated - -, 2015 that I received on - - which they said in the letter I have 10 days to respond when I received the letter 10 days from the date of the letter.