The Federal Trade Commission is investigating whether roughly a dozen prominent advertising and advocacy groups violated antitrust law by coordinating boycotts among advertisers that did not want their brands to appear alongside hateful online content, four people familiar with the inquiries said.

The inquiries include the agency’s previously reported investigation of Media Matters, a liberal advocacy organization that has published research on hateful and antisemitic content on X, the social media company owned by Elon Musk.

Ad Fontes Media, a group that rates the quality of news sources, received an investigative letter from the FTC on May 20 demanding information about its business in relation to an investigation about “possible collusion,” the organization’s CEO, Vanessa Otero, said.

At least a dozen other organizations received such letters, three of the people said. The groups include organizations that help brands choose how to position ads across the internet and watchdog groups that have sought to call attention to hateful content online.

The FTC’s new chair, Andrew Ferguson, has vowed to crack down on what he says is censorship of conservatives on social media.

Advertisers have cut or frozen spending on X and several other conservative social media platforms in recent years over worries about appearing alongside inflammatory content. They have said they should have the freedom to spend their money in line with their values.

But Ferguson has said the spending pullbacks amount to illegal boycotts.

“Drying up the advertising will dry up the idea, so the risk of an advertiser boycott is a pretty serious risk to the free exchange of ideas,” he said at a conference last month.

The FTC declined to comment Monday, citing its practice of not commenting on active investigations.

The FTC’s letter to Media Matters, which is aligned with Democrats, required the organization to share copies of its budgets, documents showing the effects of “harmful” online content on advertisers and communications with other watchdog groups.

Lawyers for groups that received FTC letters said the requests, including for information about their business practices, were broader than normal for an inquiry of this kind, according to two of the people.

Musk, who has been a close adviser to President Donald Trump, sued Media Matters in 2023 over claims that it tried to damage X’s relationship with advertisers, a lawsuit that continues. Media Matters has denied wrongdoing.

Musk has also sued watchdog organizations, advertising coalitions and individual brands over what his legal teams have described as an organized boycott against his social media platform.