I am a MD property manager - Legally, a landlord can only make deductions from your security deposit for actual damages incurred, not including normal wear and tear, such as unpaid rent and damages beyond normal wear and tear. If your landlord was a company, they should have done a walk-through of the property with you prior to you moving. If you had a private landlord, this may have been overlooked. However, Piero is correct: your landlord must provide proof of the repairs that were made in order to deduct it from your security deposit. Also, the landlord had to return your deposit within 45 days of you moving out - therefore if your letter was dated December 18 and you moved September 30, that means the landlord waited 79 days and was severely overdue. According to MD state law, if the landlord does not return a security deposit or give a statement along with proof of repairs accounting for where the security deposit was allocated to within 45 days of vacating the property, the tenant is able to file for 3x the original deposit.
I am not a lawyer, however I have years of experience as a property manager and am very familiar with the law regarding security deposits and other landlord issues. Based on what you have told me and assuming the damages incurred to the property were not beyond normal wear and tear (scuffs on the walls, worn carpet/flooring, a nick here and there), I would say you have a case against your landlord. I would strongly suggest contacting an experienced real estate lawyer with prior experience in tenant-landlord issues. A lawyer will be able to tell you in more detail what proof you will need in order to make a strong case.
Happy holidays and best of luck!