[MarkInTx]

I have looked at the picture(s) a few times. I agree with others that recommend closer examination under magnification before jumping too far. I originally thought the "line" you were referring to was the dark line between the recoil shield and the rear sight screw hole. Like dandak said...look at that one very closely also.
It will be interesting to hear what Colt says after viewing your pictures.

 

[MarkInTx]

Magnified, it does look more like a gouge or scratch than a crack.
If Colt wants to evaluate it on their dime...smart money says to let them look at it.