"We do believe they were trying to move back into Russia at least three Buk [missile launch] systems," the official said. U.S. intelligence was "starting to get indications . . . a little more than a week ago" that the Russian launchers had been moved into Ukraine, said the official.
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Aviation investigators from around the world were converging on Kiev on Saturday hoping to begin their work, but it remained unclear when they would gain full access to a mammoth site deep in rebel-held territory in the eastern part of the country. Ukrainian officials warned that the chance for an impartial inquiry was quickly slipping away as bodies were moved and at least some plane remnants were loaded onto trucks.
International observers were allowed only brief access to the site on Saturday and were restricted in their movements by the heavily armed rebels, some of whom appeared drunk, witnesses said.
"Their key task is to destroy possible evidence," said Andriy Parubiy, head of Ukraine's National Security and Defense Council. "It will be hard to conduct a full investigation with some of the objects being taken away, but we will do our best."
Ukraine and Western officials have said that Russia is providing support and equipment to the rebels.
The Kremlin has denied that it has sent weapons to the rebels, and it has continued to take a strong line against the West even after the plane crash, issuing sanctions Saturday against 13 Americans in retaliation for U.S. sanctions that were announced the day before Thursday's attack on the plane.
Ukrainian officials said that at least 38 of the 192 bodies that have been discovered had been removed from the scene and taken to the nearby rebel-held city of Donetsk.
Temperatures have been in the 80s and the bodies have been rapidly decomposing, witnesses said.
Konstantin Batozsky, an adviser to Serhiy Taruta, governor of the Donetsk region, said these actions by the rebels were meant to undermine an independent investigation and to "make all the procedures illegitimate."
Vitaly Nayda, counterintelligence chief of Ukraine's security service, offered photographs and said Ukraine has evidence of the movement of three Buk M-1 antiaircraft missile systems from rebel-held territory into Russian territory early Friday, less than12 hours after the plane was downed. Ukrainian officials have said that a missile from a Buk M-1 launcher was used to shoot down the aircraft.
Two of the antiaircraft systems were spotted entering Russia from Ukraine at 2 a.m. Friday, he said. One had its full complement of four missiles, but the other was missing a missile, he said. Two hours later, he said, a convoy of three vehicles that included one of the launchers and a control truck crossed into Russia.
The U.S. official said they could not confirm the exact time cited by the Ukrainians.
Nayda said that Ukrainian military services had not left any operational Buk M-1 launchers in territory where the rebels could have seized them when they took over bases and territory in eastern Ukraine this year. He suggested they must have come from Russia and said Ukraine has evidence that at least one launcher system was on its territory Monday.
The rebels have denied possessing the launchers, although social media files linked to a rebel leader, Igor Girkin, appeared to boast of having the systems. The claims were deleted this week after the plane was shot down.
A top rebel leader said Saturday that his side was not tampering with the evidence, even as rebels on the scene appeared to be loading at least some parts of the plane onto trucks. The leader said he was eager for international investigators to come as soon as possible.
"Currently in this area there are no active hostilities," Alexander Borodai told reporters in Donetsk. "But the situation may deteriorate at any time."
Fighting raged elsewhere in the region Saturday, especially in Luhansk near the Russian border, where 16 civilians were killed, according to the city council's website.