China’s securities watchdog said it is paying close attention to a legal case brought by Susquehanna International Group against alleged insider traders, after the US market maker said regulatory officials may have been involved in the trades.

It started with a tax bill.

China’s crackdown on cross-border trading sent the shares of popular brokers Futu Holdings Ltd. and Up Fintech Holding Ltd. plummeting, threatening pain for hedge funds and other investors who back these firms.

Trading in US-listed options on Futu Holdings Ltd. and Up Fintech Holding Ltd. surged just before China’s regulator’s announcement that it planned to penalize them triggered a record share slump.