For years, journalists have written about the social media posts of government employees to help reveal the positions, motivations and actions of public officials.

But when a journalist recently trained that same lens on Elon Musk’s new government efficiency program, the billionaire suggested that the reporting might be illegal, joining other powerful figures connected to the Trump administration who have made similar claims in recent weeks.

The offense, they argue, is doxxing: publishing private information about someone with malicious intent. The term refers to a revenge tactic, originally used by hackers, to bully, harass or intimidate people online, and can incite third parties to commit acts of violence.

Mr. Musk and others have expanded the definition this month, applying it to journalists and others seeking to hold the government accountable by reporting on public information. One Justice Department official appointed by President Trump stated early last week that he had found evidence of law breaking from people who were “targeting” employees of Mr. Musk’s government efficiency program.

Although the official did not name names, civil liberty and free speech groups said his comments appeared to refer to several journalists who had recently uncovered new details about Mr. Musk’s efforts, including identifying some of the people working for him. Those advocates say that the First Amendment explicitly protects the kind of work reporters do and that government employees are by definition not protected from critical inquiry.

Instead, they say, Mr. Musk and others are trying to intimidate and chill the media at a crucial moment.