The Wall Street Journal and New York Times both report that recent cyber attacks against US oil, gas, and power companies have come from Iran. The hackers were targeting control systems and probing for the kind of access that would give them the ability to disrupt oil pipelines and other infrastructure. The cyber attacks are likely backed by the Iranian government, though apparently the evidence isn't definitive.
Iran has apparently been stepping up its cyber warfare capabilities since its nuclear facilities fell victim to Stuxnet, widely believed to have been launched by a collaboration between the US and Israel. In fact, Iran cited attacks on its own facilities in its denial of this most recent round of hacking, telling the Wall Street Journal that "Iran has not ever retaliated against those illegal cyberattacks."
The attacks on US infrastructure are different from earlier Iranian-sourced hacks, which primarily targeted banks with less-sophisticated denial of service attacks. US officials are reportedly more concerned about the Iranian attacks than recent Chinese-based hacks, as those were primarily (though not exclusively) aimed at newspapers and collecting intellectual property.
Although it doesn't appear that the hacks have resulted in any damage, officials characterized the attacks as "probes that suggest someone is looking at how to take control of these systems," the New York Times reports. The Wall Street Journal says that although the hacks haven't yet been successful, they have gone "far enough to worry people."