brilliant round white light, larger than the following helicopter

I was inside watching cable tv.  It started having trouble with the reception when a helicopter passed loudly overhead.  I ran outside to look, since it is unusual for one to pass so low at such a late hour, although we are not all that far from Dulles Airport, and see them occasionally during the day and before 9pm.
I saw a large, round, brilliant unwaveringly bright white light, closely followed by the helicopter, which had blinking red and white lights, as seen from my angle of vision.
They were heading east in a straight-line path.  The helicopter was following closely; the object was larger than the helicopter, but made no discernable noise.  The helicopter was quite close to the object, and obviously tailing it.  I watched until they went out of sight, over the distant tree line.
I've seen other helicopters at night, even in tandem pairs, but never one following another, obviously different type of aircraft so closely.