A day after a Malaysia Airlines jetliner carrying nearly 300 people went down over Eastern Ukraine, President Obama today called the incident an "outrage of unspeakable proportions."
Obama, for the first time, disclosed that at least one American citizen was killed: Quinn Lucas Schansman. And the president, echoing what U.S. officials said earlier in the day, stated that Flight 17 was brought down by a surface-to-air missile.
"This should snap everybody's heads to attention," the president said, vowing to hold "accountable those who committed this outrage."
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On Capitol Hill, Sen. John McCain alluded to possible consequences Thursday if there is evidence of Russian involvement.
"There'll be hell to pay," McCain told ABC News.
But how will the world hold the perpetrators "accountable" and what "hell" might be paid? Here are the possible responses from the U.S. and world leaders:
Increase Pressure For A Cease-FireThis is already happening. Today, Obama said: "Russia, pro-Russian separatists, and Ukraine must adhere to an immediate ceasefire."
He added, "Evidence must not be tampered with."
The president also condemned Russia's role in the border conflict.
"The violence that's taken place there is facilitated in part, in large part, because of Russian support. And they have the ability to move those separatists in a different direction," he said.
Push For An International InvestigationThe president called for an international investigation, saying the U.S. has already offered support of the FBI and NTSB.
"Our immediate focus will be on recovering those who are lost [and] investigating exactly what happened," Obama said.
Other world leaders -- including U.S. allies like German Chancellor Angela Merkel and British Prime Minister David Cameron -- have echoed that call for an investigation.
Impose More SanctionsObama did not close the door on imposing additional sanctions against Russia.
"We will continue to make clear that as Russia engages in efforts that are supporting the separatists, that we have the capacity to increase the costs that we impose on them," he said. "And we will do so not because we're interested in hurting Russia for the sake of hurting Russia, but because we believe in standing up for the basic principle that a country's sovereignty and territorial integrity has to be respected."
Any extra sanctions would come on the heels of a new wave of sanctions announced by President Obama on Wednesday.
"These sanctions are significant but they are also targeted, designed to have the maximum effect on Russia while limiting any spillover effects on American companies," the president added. "The Russian leadership will see once again that its action has consequences."
The sanctions unveiled earlier this week targeted Russian banks and energy and defense firms.
Get The U.S. Military InvolvedThis option is not likely -- at least not right now. The president was clear in his remarks at the White House on Friday: "We don't see a U.S. military role beyond what we've already been doing."
But on Thursday, McCain did suggest the U.S. should provide arms to the Ukrainian military if Russian involvement is proved.
Ask Europe To Take the LeadThe U.S. may encourage European leaders to respond in a stronger manner to any possible Russian involvement in the downing of the Malaysian plane.
"This certainly will be a wake-up call for Europe," Obama said.
Former Secretary of State Hillary Clinton suggested on Thursday that European leaders should take the lead on responding to any Russian involvement.
"The Europeans have to be the ones to take the lead on this," Clinton said in an interview with PBS' Charlie Rose Thursday. "It was a flight from Amsterdam to Kuala Lumpur, over European territory. There should be outrage in European capitals."
European leaders will likely be asked to join the U.S. in imposing tougher sanctions against Russia. Additionally, Clinton said the European countries should increase their efforts to find alternatives to Gazprom, a Russian entity which provides 30% of Europe's natural gas.
Ramp Up DiplomacyThis all depends on how the investigation unfolds and how Russian President Vladimir Putin responds.
The U.S. is already in steady contact with Ukrainian officials, but there is a chance diplomatic channels could open up between Russia and Ukraine -- if Putin is willing to come to the table.
Prior to the plane incident, Obama said he told Putin the United States' "preferred path is to resolve this diplomatically" and made clear that the Russian government needed to "make a strategic decision."
"Are they going to continue to support violent separatists whose intent is to undermine the government of Ukraine, or are they prepared to work with the government of Ukraine to arrive at a cease-fire and a peace that takes into account the interests of all Ukrainians?" Obama said.
Do NothingSome believe the European Union may not do that much in response to the downed jetliner. European leaders might decide that any tough sanctions imposed against Russia might negatively affect Europe's own economy.
"Unless they discover a sanction that punishes Russia without punishing Europeans - and the answer is they can't - I think the EU is going to show its mettle," Graham Allison, a professor at Harvard's Kennedy School of Government and a former assistant secretary of defense under President Bill Clinton, told Politico.
ABC News' Erin Dooley and Mary Bruce contributed to this report.
How the World Might Respond to Malaysia Plane Disaster