A federally appointed overseer at HSBC Holdings PLC flagged suspicious transactions in the accounts of Huawei Technologies Co. to prosecutors seeking the extradition of the Chinese company’s finance chief, people familiar with the matter said.

A monitor charged with evaluating HSBC’s anti-money-laundering and sanctions controls in recent years relayed information about the Huawei transactions to federal prosecutors in the Eastern District of New York, the people said.

Canadian...