When the two top executives of a hot electric-vehicle startup made share purchases that later drew scrutiny, they were helped by accounting firm BDO USA, according to the auto company. BDO was also the auditor of the company they ran.

The dual roles that BDO played at Electric Last Mile Solutions are typical of potential conflicts of interest faced by auditors. Relationships like this one are under scrutiny by the Securities and Exchange Commission, people close to the inquiry said.