They wouldn't even need to do that. Anyone that was in the DC area during the DC Sniper Attacks will remember the mass hysteria two a-holes in a caprice caused the whole area. Everyone was looking for a white box truck because some news channel said they were using it (nope), and gas stations put up sheets or other barricades to hide people while they were refueling at their pumps.

I had to go to DC on business during that time and my whole family was freaked out and just knew I would get shot.

Two guys with a gun and a plan were able to pretty much control a metro area. The same could happen in a bunch of metro/urban areas because those areas still have anti-gun laws that keep the number of "good guys with a gun" to a bare minimum.

Honestly, if we start seeing power plants, substations, water and/or sewage treatment plants, or other infrastructure being sabotaged or attacked I would suspect a state actor, or at the minimum some terrorist group. Regardless of how crazy some of these gunmen are, unless they want to die on the first crime I would think they would want their rampage to continue unabated. Knocking out critical infrastructure would bring the whole state and federal government down on them immediately. Not saying it is impossible, but I know if I started seeing attacks on critical infrastructure I would start making plans in case China or someone similar is getting ready for an attack.