Not a lawyer, and never done this, but the law is pretty clear about security deposits and I'd imagine you could probably easily pursue a case in small claims at this point. Small claims cases are relatively informal and you do not generally need legal representation. You may also be referred to a small claims mediator prior to trial that may negate the need to have an actual trial too. Here's the DC Court's page about the process and how to file a case.

One final thing you may want to do if you think small claims is the route you want to take, is send your landlord a certified letter telling them they have 7 days from receipt to send return your deposit or that you will be filling in small claims. I only suggest this as threat of legal action can light a fire under people's asses and could avoid the rigamarole of small claims, but this isn't totally necessary either. If you do decide to do this and they don't respond to that letter, well you sent it certified and now have additional evidence that they knew they were illegally withholding your deposit.