As the US government investigates Facebook on suspicion of antitrust violations, some of Facebook's biggest rivals are said to be getting involved.

Snap, the company behind Snapchat, is reportedly working with the Federal Trade Commission on its investigation — and a dossier documenting what it considers anticompetitive behavior from the social-media giant could be part of that cooperation.

The dossier's name is "Project Voldemort," according to a new report from The Wall Street Journal's Georgia Wells and Deepa Seetharaman.

Lord Voldemort is the bad guy in the "Harry Potter" books and movies. Warner Brothers

Facebook is known to replicate competitors' features for its namesake social-media platform as well as for Instagram and WhatsApp. Instagram Stories, for instance, is widely seen as a rip-off of Snap Stories. Camera filters on Facebook were pioneered on Snapchat. The list goes on and on.

Read more: Facebook says the FTC is officially investigating it over antitrust concerns

Snap was reportedly documenting those instances in its "Project Voldemort" dossier — it's unclear whether or not Snap has handed over that dossier to the FTC.

Snap isn't the only one working with the FTC on its investigation of Facebook: The Journal report said the agency was in touch with "executives from startups that became defunct after losing access to Facebook's platform" and "founders who sold their companies to Facebook."

Representatives for Snap and Facebook didn't respond to requests for comment as of publishing, and representatives for the FTC declined to comment.