Evidence collected by the U.S. Department of Homeland Security suggests that cyber attacks on key energy infrastructure—and on the electricity system — are increasing, both in frequency and sophistication…
Read more: http://bipartisanpolicy.org/events/2014/02/cybersecurity-and-north-american-electric-grid-new-policy-approaches-address-evolving

Commander: U.S. Military Not Ready for Cyber Warfare

Unfortunately, there were not only cyber “attacks on key energy infrastructure”:

Sniper Attack [in California] Knocked Out Substation, Raises Concern for Country’s Power Grid. The incident happened one day after the Boston bombings which is why there’s been little reporting on the incident. Now authorities say the damage is the work of a sniper and they are treating the shootings an act of terror. “It appears to be very well organized and very well planned,” said Steve Harrison of the Arizona Department of Public Safety. In just two hours in the middle of an April night last year, a militarystyle sniper attack took out 17 transformers powering the Silicon Valley… Security expert Norman Beasley says power stations can easy pickings for bad guys because they are out in the open… The FBI continues to investigate the attack, but no arrests have been made.
~California power station attack last April was “act of terror”.

See also:

Blackout a reminder of power grid vulnerabilities. Associated Press. SAN DIEGO — A blackout that swept across parts of the Southwest and Mexico apparently began with a single utility worker and a minor repair job. How it then rippled from that worker in the Arizona desert, to southern California and across the border, plunging millions of people into darkness, has authorities and experts puzzled, especially since the power grid is built to withstand such mishaps. However it … was a reminder that the nation’s transmission lines remain all too vulnerable to cascading power failures. “There are a lot of critical pieces of equipment on the system and we have less defense than we think,” said Rich Sedano at the Regulatory Assistance Project, a utility industry think tank based in Montpelier, Vt. … There have been several similar failures in recent years.

U.S. Risks National Blackout From Small-Scale Attack. Federal Analysis Says Sabotage of Nine Key Substations Is Sufficient for Broad Outage. The U.S. could suffer a coast-to-coast blackout if saboteurs knocked out just nine of the country’s 55,000 electric-transmission substations on a scorching summer day, according to a previously unreported federal analysis… “This would be an event of unprecedented proportions,” said Ross Baldick, a professor of electrical engineering at the University of Texas at Austin… Sabotage could wreak worse havoc, experts said. “The power grid, built over many decades in a benign environment, now faces a range of threats it was never designed to survive,” said Paul Stockton, a former assistant secretary of defense and president of risk-assessment firm Cloud Peak Analytics.

National Research Council worried that a well-coordinated attack on the grid “could deny large regions of the country access to bulk system power for weeks or even months.

Electric Grid Vulnerable to EMP, Could kill 9 in 10 Americans