Hanlon's razor: "Never attribute to malice that which is adequately explained by stupidity."
Next, bullet holes in rural road signs will be evidence of domestic terrorism.
"This wasn't an incident where Billy-Bob and Joe decided, after a few brewskis, to come in and shoot up a substation," Mark Johnson, retired vice president of transmission for PG&E, told the utility security conference, according to a video of his presentation. "This was an event that was well thought out, well planned and they targeted certain components." When reached, Mr. Johnson declined to comment further".
-- "Assault on California Power Station Raises Alarm on Potential for Terrorism", Wall Street Journal. http://online.wsj.com/news/articles/SB1000142405270230485110...
And I would totally buy the explanation of drunk dumbass. The only thing that suggests something more sinister is that the attack was preceded by someone entering a vault and cutting telephone wires.
That suggests to me more of a "probing" to see just how secure these facilities are. With that in mind, it might not be an actual terrorist cell, but some big agency testing out the response of others. Speculation, but it makes sense for the FBI to look deeper into this.