I guess they could break the job down: use one team of 4 drones to lift multiple 1 square yard tarps.
Let's see, 12 square miles is roughly 37 million square yards. Multiplying by four they would only need about 150 million drones. Quite affordable for today's well-funded terrorist with an Amazon Prime account (as long as they choose 3-5 business day shipping instead of overnight).
https://www.tarpsplus.com/blue-poly-tarp-sold-individually.h...
"3.5 oz. Per Square Yrd."
So each drone should be able to lift about a gigagram.(Also, this plastic doesn't have enough tensile strength to support its own weight when lifted from the corners.)