Facebook Inc. for most of the past decade was Silicon Valley’s 800-pound gorilla, squashing rivals, co-opting their best ideas or buying them outright as it cemented its dominance of social media.

Now the knives are coming out. A number of Facebook’s current and former competitors are talking about the company’s hardball tactics to investigators from the Federal Trade Commission, as part of its broader antitrust investigation into the social-media giant’s business practices, according to people familiar with the matter.

One of them is Snap Inc., where the legal team for years kept a dossier of ways that the company felt Facebook was trying to thwart competition from the buzzy upstart, according to some of those people. The title of the documents: Project Voldemort.

The files in Voldemort, a reference to the fictional antagonist in the popular Harry Potter children’s books, chronicled Facebook moves that Snap officials believed were a threat to undermine Snap’s business, including discouraging popular account holders, or influencers, from referencing Snap on their accounts on Instagram, which Facebook owns, according to people familiar with the project. Executives also suspected that Instagram was preventing Snap content from trending on its app, the people said.

In recent months, the FTC has made contact with dozens of tech executives and app developers, people familiar with the agency’s outreach said. The agency’s investigators are also talking to executives from startups that became defunct after losing access to Facebook’s platform in addition to founders who sold their companies to Facebook, according to some of those people.