That is where HSBC came in.

Since at least 2009, Huawei had been a client of the bank. In 2013, a report by Reuters revealed that Huawei, through a subsidiary called Skycom, had been secretly doing business in Iran — a country subject to stringent international sanctions.

HSBC asked Huawei if the Reuters report was true. It was a big concern for the bank, because the previous year federal prosecutors had accused it of willfully failing to stop money laundering by customers, including in countries like Iran. To settle that investigation, HSBC had paid a $1.9 billion fine, entered into a deferred prosecution agreement and agreed to have a court-supervised monitor installed inside the bank.

While American companies are legally restricted from doing business with people and institutions in countries under United States sanctions, nothing stops Huawei from doing so. But if an international bank like HSBC moves money through the United States on its way to or from Iran, that potentially violates American law — which is what HSBC executives were apparently worried about with Huawei and Skycom.

Huawei sent Ms. Meng — a daughter of Huawei’s founder, Ren Zhengfei — to try to assuage HSBC’s concerns. In August 2013, she gave a PowerPoint presentation to HSBC officials in which she denied that Huawei was connected to Skycom. She also said Huawei had complied with American sanctions and would not use HSBC for any transactions in Iran, according to Canadian court filings.

While Ms. Meng’s presentation was in Chinese, Huawei later sent an English version to the bank. “HSBC has been working with Huawei for a long time and has a deep understanding of Huawei’s history of growth around the world,” the presentation said. “We have been and will continue to be transparent.”

The presentation was kept on file at HSBC. Several of the bank’s risk-assessment committees relied on it to justify continuing to do business with Huawei. By 2014, the bank had unwittingly cleared more than $100 million in transactions with Skycom in Iran, according to federal prosecutors.