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The United States has found no direct link between the Russian government and the downing of Malaysia Airlines Flight 17 last week, three senior intelligence officials said Tuesday.

The officials repeated their belief that pro-Russian separatists fired the missile that took out the jetliner over Ukraine, killing 298 people, but they said they could not say definitively that the separatists did it.

“There’s not going to be a Perry Mason moment,” one U.S. official said. “We don’t know who pulled the trigger.”

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The White House stressed that it was not backing off what President Barack Obama, Secretary of State John Kerry and U.N. Ambassador Samantha Power have said for days — that Russia provided training and support for the separatists who probably shot down the plane, and therefore bore some responsibility.

Instead, the intelligence officials were “just saying that we don’t know if Russians pulled the trigger,” one administration official said. “That doesn’t absolve Russia.”

The officials cited several reasons they could not make a final determination about who fired. One was that U.S. officials have yet to see the plane wreckage in person.

The disclosures came a day after Russian officials challenged the United States to back up its finger-pointing with evidence.

The intelligence officials, summarizing the findings of American intelligence five days after the crash, said the most likely explanation was that the separatists hit the passenger jet by mistake. Separatists had boasted about shooting down Ukrainian military aircraft.

And the intelligence officials said that Russia is still helping the separatists. As recently as Tuesday, the Ukrainian military told the United States that at least 20 vehicles, including tanks and armored vehicles, crossed from Russia into Ukraine.

Russia also stepped up its deliveries of vehicles and rocket launchers in the weeks prior to the downing of Flight 17, one official said, noting that the Ukrainian military was making advances at the time and Russia was trying to help the separatists fight back.

Russia has been “very deliberate” about the equipment it ships, the official said, and sometimes sends the same equipment that the Ukrainian military uses so that Russia can deny knowing where it came from. The official even cited instances in which Russia has sent tanks no longer in service in Russia to the separatists so they have the same models as the Ukrainian military.

The United States believes that the separatists had at least one SA-11 missile launcher at the time of the crash, but the officials said that the U.S. did not know they had it until after Flight 17 was shot down. The SA-11 missile is believed to have taken out the plane.

Andrea Mitchell of NBC News contributed to this report.