After a little searching I came up with this
http://weblinks.westlaw.com/toc/defa...nt#DC014798272. It's the Vehicle Code for D.C. Scroll down to section 50, Abandoned Vehicles. I read through it and it seems that the Department of Public Works is responsible for the towing, storage and auction of abandoned vehicles. Here's a link to them.
http://www.dpw.dc.gov/dpw/site/defau...dpwNav=|31200|
I'd say you best bet is to contact them directly. I didn't catch whether you owned the property or not. If you do, I believe you can lay claim to these cars. It's in the code somewhere.