US intelligence officials say that a surface-to-air missile did indeed bring down a passenger jet over Ukraine today, but they can't say who fired it or even which side of the border it came from, reports the Wall Street Journal. Both the Ukraine government and pro-Russian separatists have denied responsibility. A Ukraine official asserts that a missile system known as a Buk launcher brought down the plane, and he alleges that Russia supplied it to separatists. The AP reports seeing something similar to a Buk system this week in separatist-held territory, but that's not exactly damning proof—a weapons expert points out that Ukraine forces also have Buk systems.
It will take a while for investigators to make an official determination, though CNBC cites unconfirmed reports that the plane's black-box data recorder has been found, which should speed things up. Meanwhile, Joe Biden, who said the plane was "blown out of the sky," spoke to Ukraine President Petro Poroshenko and offered US help with the investigation, reports Reuters. Poroshenko is calling the plane crash a "terrorist act," notes the New York Times, though President Obama used the phrase "terrible tragedy" to describe it in his initial comments. No word yet on whether any Americans were among the 298 people aboard. All are feared dead. (Read more Ukraine stories.)