The United States detected the launch of the “specific missile” that brought down Malaysia Airlines Flight 17 last week, a senior administration official told reporters on Tuesday.

U.S. intelligence followed “this specific missile” as it was fired from “a geographic area” controlled by Russia-backed separatists in eastern Ukraine, said the official, who requested anonymity. It followed the near-vertical flight path characteristic of an SA-11 surface-to-air missile launch.

“We did pick up a launch. We were able to have the ability to track this specific launch,” the official said. It was not clear whether the official was referring to real-time monitoring, or whether U.S. intelligence had gone back through surveillance data after learning of the attack.

The official spoke as the United States ramped up efforts to convince skeptics that Moscow-backed rebels armed and trained by Russia shot down the passenger jet, killing all 298 people aboard. Russia has disputed the largely circumstantial American case and rejected responsibility.

“We don’t know who, like, pulled the trigger,” the official said. “We don’t know that a Russian was operating the system. However, that in no way precludes Russian involvement in the sense that they are the most likely source for this system.”

“We assign responsibility generally to Russia for how the separatists are armed and trained,” the official said. “What we have is a kind of picture of evidence that says the Russians have been providing these arms, these types of systems, and the Russians have been providing training — and that adds up to a picture that implicates Russia,” but “we’re going to be careful about saying who did it if we don’t know for certain.”

“We don’t know who literally was operating the system that day,” the official said. “That’s the hardest thing to determine.”

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