News of the investigation, which reportedly involved a subpoena to ByteDance along with a number of interviews by the FBI, comes after TikTok confirmed in December that four ByteDance employees had been fired in connection with the incident, following an internal review.
Two of the fired employees had been based in China, and two were based in the United States, TikTok said at the time. Among the TikTok users who were surveilled were two journalists, including the Forbes journalist who on Friday first reported the Department of Justice probe.
The DOJ, FBI and the US attorney for the Eastern District of Virginia, who is also reportedly involved in the probe, declined to comment. TikTok did not respond to a request for comment.
The reported inquiry reflects the mounting pressure on a company that US officials say poses a national security risk. Policymakers fear the Chinese government could pressure TikTok or its parent to hand over the data TikTok collects on its US users. The concerns have sparked widespread bans of TikTok on official government devices in the United States, the European Union, Canada, the United Kingdom and New Zealand.
The Biden administration has gone a step further, threatening TikTok with a nationwide US ban unless its Chinese owners sell their shares in the popular social media app, which is used by more than 100 million Americans.
The surveillance that led to the firings saw ByteDance employees accessing device information such as IP addresses used by the journalists. The initial reports about the incident suggested the employees had been hunting for the source of leaks to the press. There is currently no evidence the Chinese government directed or participated in the surveillance.