The government has made representations that we're facing a "cyber warfare" threat, and NSA is a paramilitary organization with a big role to play in conducting such a conflict.
The FBI is an investigative agency. They aren't beat cops. They aren't soldiers. They do have a counter-intelligence role.
From a threat POV, consider how dependent on gasoline and diesel the US economy is, and what the impact of disruption would be. If an attacker could disrupt fuel supplies in key areas or highway corridors, they would cause all sorts of chaos and economic damage.
As Americans, we've been very lucky to have two oceans protect us from the shift in warfare from the battlefield to total war, with the exception of the US Civil War. Our global connectivity changed the threat landscape. That's not a police matter.