“What I am concerned about and what I think we really need to be concerned about is when these transition from disruptive to destructive attacks, and I think those are coming,” Alexander said during remarks Monday to the American Enterprise Institute.

Such attacks are capable of destroying key elements of a system to the point where the equipment cannot be repaired and must be replaced, such as if a computer’s Basic Input Output System, or BIOS, or other system software is destroyed, he said…

Deterring such attacks has grown more difficult because many attacks are not “nation-on-nation” strikes.

Some attacks are from unknown origins, but “either way, the outcome could be the same: You lose the financial sector or the power grid or your systems capabilities for a period of time. It doesn’t matter who did it; you still lose that. So you’ve got to come up with a defensive strategy that solves that, from my perspective.”