Were they big and loud (or so far away you couldn't tell)? If so, I often see Army (or Marine or ...) troop copters (like the double-rotor Chinooks) flying in convoys. Then again, we have more military bases down here around DC than you do up in Baltimore.
None looked that big, and none was a double-rotor. The different-shaped one was a little larger than the other three, I think, but they all seemed to be (as far as I could judge from that distance) to be comparable in size to the police helicopter ... maybe a little bigger.
Helicopters
Re: Helicopters