WASHINGTON (AP) — The United States began building a case Friday linking pro-Russian separatists to the shocking downing of a passenger jet in Ukraine. A somber President Barack Obama declared the deaths of those on board, including at least one American, an “outrage of unspeakable proportions.”

Obama said the U.S. believes the Malaysia Airlines plane was felled by a surface-to-air missile launched from an area near the Ukraine-Russia border that is controlled by Kremlin-backed separatists. Even as he cautioned that the exact circumstances were still being determined, the president turned his sights on Russia, saying the insurgents would not be capable of carrying out such an attack without Moscow’s support.