Citibank Debt Collection Complaint

Citibank Debt collection department,

Federal student loan Cont'd attempts collect debt not owed Debt resulted from identity theft Maryland

I had a student loan back in 2002, was finally paid off this summer and now I get a statement from citibank/- - - saying I now owe $18000.00 dollars, which is entirely false, I never owe anything close to that. I do some research and found out that Citibank/- - - may be a Scam company. Please help me because they are threatening to sue me by another outside lawyer firm called - - - - - , - . Their infomation is- - - - - - , - . - - -, - . - -, MD - - thank you in advance for any help you can lend to me.