The data are less clear on which countries were the main culprits in the $41.1 billion of net sales. While holdings attributed to China, the largest foreign owner of Treasurys, rose in August by $1.7 billion to $1.27 trillion, those in Belgium plunged by $44.8 billion to $110.7 billion.

Belgium has been regarded by analysts including David Woo of Bank of America Corp. as a transit point for China’s transactions involving Treasurys. The U.S. Treasury data include a disclaimer that a nation’s assets in a custodial account in a third country won’t reflect the true ownership of securities, so looking at holdings attributed to individual nations can be misleading.

“There’s no other buyer that would have the incentive to go through Belgium,” said Aaron Kohli, a fixed-income strategist at Bank of Montreal, one of 22 primary dealers that trade with the U.S. central bank.

“There’s no definitive link there, but a lot of what people glean is from the sheer size of the holdings. Definitely the decline suggests there’s been some significant active selling.”

Attributing the Belgium sales to China would make more sense, because the world’s second-biggest economy has been selling U.S. debt to support the yuan after a surprise devaluation spurred bets on a weaker currency.

Bloomberg News reported in August that China cut its holdings of U.S. Treasurys that month to raise dollars needed to support the yuan. Channels for such transactions include China selling directly, as well as through agents in Belgium and Switzerland, said a person familiar with the matter, who asked not to be identified as the information wasn’t public.

Global Turmoil

The data flesh out some of the story behind the month’s global financial turmoil that contributed to the Federal Reserve’s September decision to refrain from raising interest rates. With China’s economy weakening and the greenback up 19 percent in a year against a basket of currencies, the People’s Bank of China on Aug. 11 unexpectedly devalued the yuan, resulting in its sharpest decline since 1994.

China had previously reported a record $93.9 billion drop during August in the nation’s foreign-exchange reserves, and the country’s stock-market plunge later in the month sparked a worldwide selloff. The reserves dropped another $43.3 billion in September to $3.51 trillion, bringing the decline to $479 billion since the peak in mid-2014.

Private foreign investors bought a net $6.2 billion of Treasury bonds and notes in August, resulting in net foreign selling of about $35 billion when including the official sales.

The Treasury’s report, which also contains data on international capital flows, showed net foreign purchases of long-term securities of $20.4 billion in August. It showed a total cross-border outflow, including short-term securities such as Treasury bills and stock swaps, of $9.2 billion.