The order came down from the highest levels of China's government, in a handwritten message from President Xi Jinping to officials charged with fixing the country's crashing stock market: Make sure to protect the interests of small and mid-level investors.

Xi's command — scribbled across a report on probes into short sellers and other "malicious" traders — was shared at a July meeting between regulators and law enforcement officials, according to a person with knowledge of the matter, who asked not to be named because the meeting was private.

In the ensuing months, policy makers would escalate an unprecedented campaign to prop up shares and punish speculators accused of exacerbating the rout.