However, Marc Chandler, chief Treasury strategist at Brown Brothers Harriman, said everything points to Russia, starting with the timing of Sunday's Crimean referendum and the potential for Western sanctions. He said rather than selling the Treasurys, Russia simply transferred them out of the U.S.
"Everything (Russian President Vladimir) Putin is doing is being extra cautious about retaliation, like bringing troops right to the Ukraine border," said Chandler. "The other reason I say it's most likely Russia is you look at the countries that have the largest reserves. It doesn't feel like it's China because China has enough on its plate."
Chandler said about half of Russia's foreign currency reserves are held in dollar-denominated instruments, and this would be about 80 percent of those dollar holdings. At the end of 2013, Russia had $138 billion in Treasury securities.
"It could be somebody else, but it does seem circumstantial just on the timing alone," said Chandler.
He also said there are precedents for this type of behavior by Russia.
He pointed to the birth of the Eurodollar market—dollars outside the U.S. In 1956, when the Soviet Union invaded Hungary, it feared the U.S. would retaliate with financial muscle, and Russia-based Narodny Bank shifted dollars from the U.S. and deposited them in its London branch, he noted.