Investors wrestling with the mysterious U.S. bond rally of 2014 got a clue about where to look: China.

The Chinese government has increased its buying of U.S. Treasurys this year at the fastest pace since records began more than three decades ago, data released Wednesday show. The purchases help explain Treasurys' unexpectedly strong rally this year. The yield on the 10-year U.S. Treasury note has fallen to 2.54%, from 3% at the end of 2013. Yields fall as prices rise.

The world's most-populous nation boosted its official holdings of Treasury debt maturing in more than a year by $107.21 billion in the first five months of 2014, according to the U.S. government data. The buying has been fueled by China's efforts to lift its export-driven economy by weakening its currency, the yuan, against the dollar, market analysts said, a strategy that encompasses hefty purchases of U.S. assets.

China officially holds roughly $1.27 trillion of U.S. debt, about 10.6% of the $12 trillion U.S. Treasury market.

The country's purchases have salutary effects on both sides of the Pacific. In addition to the weaker yuan in China, they hold down U.S. interest rates, making houses more affordable and generally easing financial conditions in the U.S. economy.