(AP) - Senior U.S. intelligence officials said Tuesday that Russia was responsible for "creating the conditions" that led to the shooting down of Malaysia Airlines Flight 17, but they offered no evidence of direct Russian government involvement.

The intelligence officials were cautious in their assessment, noting that while the Russians have been arming separatists in eastern Ukraine, the U.S. had no direct evidence that the missile used to shoot down the passenger jet came from Russia.

The officials briefed reporters Tuesday under ground rules that their names not be used in discussing intelligence related to last week's air disaster, which killed 298 people.

The plane was likely shot down by an SA-11 surface-to-air missile fired by Russian-backed separatists in eastern Ukraine, the intelligence officials said, citing intercepts, satellite photos and social media postings by separatists, some of which have been authenticated by U.S. experts.

But the officials said they did not know who fired the missile or whether any Russian operatives were present at the missile launch. They were not certain that the missile crew was trained in Russia, although they described a stepped-up campaign in recent weeks by Russia to arm and train the rebels, which they say has continued even after the downing of the commercial jetliner.

In terms of who fired the missile, "we don't know a name, we don't know a rank and we're not even 100 percent sure of a nationality," one official said, adding at another point, "There is not going to be a Perry Mason moment here."

The officials said the most likely explanation for why the plane was shot down is that the rebels made a mistake. Separatists previously had shot down 12 Ukrainian military airplanes, the officials said.

The officials made clear they were relying in part on social media postings and videos made public in recent days by the Ukrainian government, even though they have not been able to authenticate all of it. For example, they cited a video of a missile launcher said to have been crossing the Russian border after the launch, appearing to be missing a missile.

But later, under questioning, the officials acknowledged they had not yet verified that the video was exactly what it purported to be.

On Monday, President Barack Obama said the Malaysia Airlines plane "was shot down over territory controlled by Russian-backed separatists in Ukraine." He also said Russia has both trained the separatists and "armed them with military equipment and weapons, including anti-aircraft weapons."