Exclusively online degrees in the education field are near impossible, especially if you want to be certified in the elementary grades because most states require practicums/internships, and student teaching in the schools to be certified at that grade level.
My Bachelors degree is in Early Childhood, and my Masters degree will be in Special Education. Even though I did student teaching for my Bachelor's degree, I'm required to have at least a practicum experience to graduate.
You'll also have to take the PRAXIS tests for licensure. There's the PRAXIS I (3 tests) that everyone has to take, and the PRAXIS II (tests depend on area of certification and state)
There are tons of colleges in the DC area, the best thing you can do is check around for schools near you and see what programs they offer and if any components are online. I know some programs offer 50/50 in person and online classes, etc. You can check American University, University of MD, Loyola, Hood, McDaniel College, just to name a few.
If your researching online programs, always check to make sure the program is accredited before you sign up. Some programs advertise that they are certified (like University of Phoenix), but the the your state's education department. may not offer you a teaching licensure depending on what there licensure requirements are. As well as researching colleges I would encourage you to visit your state's department of education to check their licensure requirements.
Here's MD:
http://marylandpublicschools.org/MSDE/divisions/certification/?WBCMODE=Presentatio%25%3e%25%3e%0d
Virginia:
http://www.doe.virginia.gov/teaching/index.shtml
DC:
http://www.ed.gov/category/location/district-columbia