The managing director of a British energy company, believing his boss was on the phone, followed orders one Friday afternoon in March to wire more than $240,000 to an account in Hungary, said representatives from the French insurance giant Euler Hermes, which declined to name the company.

The request was “rather strange,” the director noted later in an email, but the voice was so lifelike that he felt he had no choice but to comply. The insurer, whose case was first reported by the Wall Street Journal, provided new details on the theft to The Washington Post on Wednesday, including an email from the employee tricked by what the insurer is referring to internally as “the false Johannes.”

Now being developed by a wide range of Silicon Valley titans and AI start-ups, such voice-synthesis software can copy the rhythms and intonations of a person’s voice and be used to produce convincing speech. Tech giants such as Google and smaller firms such as the “ultrarealistic voice cloning” start-up Lyrebird have helped refine the resulting fakes and made the tools more widely available for free and unlimited use.