WASHINGTON — The United States began building a case Friday that would pin the blame for the downing of the passenger jet over Ukraine on separatist forces supported by Russia. President Barack Obama said one American was among the nearly 300 killed in an disaster that could dramatically escalate the crisis in Ukraine.

Obama said evidence so far indicates that Malaysia Airlines Flight 17 was shot down by a surface-to-air missile from an area controlled by Russian-backed separatists. He noted it wasn’t the first time the separatists had shot down planes in the region, adding that a “steady flow of support from Russia” had included heavy weapons and anti-aircraft weapons.

He called for an immediate cease-fire to allow for a full investigation

“This was a global tragedy,” Obama said. “An Asian airliner was destroyed in European skies filled with citizens from many countries, so there has to be a credible international investigation into what happened.”

Officials from the FBI and the National Transportation Safety Board were on their way to Ukraine to help determine what happened, Obama said. He warned that evidence must not be tampered with as a United Nations-backed investigation goes forward, and he said, “We will hold all its members, including Russia, to their word” in allowing access to the crash.

“This should snap everybody’s heads to attention,” Obama said.