Elon Musk’s Twitter sparked an international outcry on Thursday by suspending a number of journalists at major news organizations who cover him. But a different and possibly related move that Twitter took around the same time, against a fast-growing rival, could open the company up to regulatory scrutiny, some legal experts say. In addition to suspending journalists who had been covering a controversy related to the third-party tracking of Musk’s private jet, the platform also suspended the official Twitter account of rival Mastodon after it tweeted about the ElonJet account. Twitter users began tweeting links to their Mastodon profiles, some half-jokingly telling followers where they could be found on the alternative platform in case they, too, were banned from Twitter without warning. Soon, however, Twitter began throwing up roadblocks — marking links to Mastodon as “unsafe” and potentially malicious, blocking tweets containing those links and preventing users from adding Mastodon links to their profiles. Now, legal experts are pondering whether there may be anticompetitive or other regulatory implications arising from Twitter’s blocking of Mastodon links. “You could see all sorts of problems, both from a competition and a consumer protection standpoint,” said Bill Baer, who has served as the former top antitrust official at the Justice Department and at the Federal Trade Commission across two separate US administrations. Those questions, which bring to bear on Twitter some of the antitrust scrutiny that’s been leveled at larger tech giants Meta and Google, come just as Twitter faces intensifying questions about its ability to comply with a US government consent decree — along with concerns about hate speech on the platform and the possible precedent set by its suspension of journalists reporting on Musk. Twitter, which has cut much of its public relations team, didn’t respond to a request for comment. Twitter’s restrictions on Mastodon As news of the journalist suspensions spread, many Twitter users announced they were migrating or expanding to Mastodon. But the sudden restrictions Twitter placed on link-sharing appeared to thwart some attempts at referring users to the alternative platform. “Twitter is now attempting to prevent its users from navigating to the official social media accounts of elected officials on other platforms,” said Virginia Democratic Rep. Don Beyer, sharing a screenshot of a Twitter system message warning that Beyer’s link to his Mastodon profile was “potentially spammy or unsafe.” Other users, such as New York Times editor Patrick LaForge, observed that attempts to add Mastodon links to Twitter profiles were producing error messages from Twitter warning that the links were “considered malware.” CNN confirmed some of the reports with its own testing, finding that Twitter blocked attempts to tweet links referring users to a Mastodon profile. Sharing Mastodon user handles as plain text, and using link-shortening services that obscured the destination URL, allowed users to get around the restrictions. But the link blocking continued into Friday afternoon. Musk has falsely claimed that the suspended journalists shared real-time information on his physical location, violating Twitter policies. After one of the suspended reporters challenged Musk’s claim in a Twitter Spaces event that Musk spontaneously dropped in on Thursday evening, the new Twitter owner abruptly left the conversation. Eugen Rochko, the founder and CEO of Mastodon, has not publicly addressed Twitter’s link blocking, but has amplified a public report about it. CNN has reached out to Rochko for comment. A fast-growing rival While there are some differences in how the two platforms operate, Mastodon’s user experience replicates much of the basic Twitter functionality. Twitter is much larger, boasting some 238 million users to Mastodon’s 2.5 million, but the latter has grown rapidly since Musk acquired Twitter. In the first week and a half after Musk closed his Twitter deal, Mastodon gained hundreds of thousands of users, and the migration has only continued since then. Twitter’s move to block links to a nascent rival could be the type of activity to pique the interest of the Federal Trade Commission, whose chair, Lina Khan, has vowed to crack down on novel ways in which tech platforms might try to hurt competition. If regulators could prove Twitter intentionally used the link blocking to preserve a form of market dominance and to keep a potential rival at bay, then they might have a case, legal experts say. In general, companies aren’t obligated to do business with each other and can freely choose their business partners. But a dominant firm that’s said to have “market power” can potentially violate antitrust law if they refuse to deal with other parties. This notion of a “duty to deal” is likely the most relevant to this situation, according to Charlotte Slaiman, competition policy director at the consumer advocacy group Public Knowledge and a former FTC antitrust official. “If Twitter has market power, it may have some duties to deal with competitors,” Slaiman said. “Duties to deal is an area of antitrust law that I think is really important in the tech sector, but has been aggressively narrowed” in recent decades. Under Khan, a vocal tech skeptic, the FTC has shown an expanded interest in duty-to-deal cases through recent policy statements, Slaiman added. And during the Trump administration, the FTC alleged Facebook had acted anticompetitively by effectively blocking access to Vine, a Twitter-owned video platform, as part of a wider lawsuit seeking to break up the social media giant. (The FTC complaint was later thrown out by a federal judge, but re-filed with slightly different arguments on Khan’s watch.) A potentially complicated case A case revolving around duty to deal would likely need to argue that Twitter had harmed itself somehow by restricting Mastodon link-sharing — perhaps by making itself less likely to receive incoming traffic from Mastodon, or by making itself less attractive to advertisers as an open platform. Simultaneously, it would also likely need to show that Twitter’s actions hurt Mastodon even worse, by taking something critical away from it (in this case, potentially, an influx of new users). Before that, however, a judge would first have to agree that Twitter enjoys “market power,” or dominance in a specific market that regulators would be expected to describe and explain in any lawsuit. That definition could take a variety of forms, but it would need to pass muster with the judge before prosecutors would even have a shot at arguing that Twitter’s conduct was anticompetitive. It could be a tough case, Baer and Slaiman said. Baer added that Twitter’s link blocking doesn’t just raise potential competition concerns. It also raises questions about Twitter’s stated reasons for blocking the links, and whether those public justifications hold up to scrutiny by consumer protection officials. As Beyer tweeted, the link he shared to his own Mastodon profile was not malicious. And prior to Thursday, there did not appear to be grounds for Twitter to claim that Mastodon links were unsafe. If Twitter misled the public with its statements about Mastodon links, saying they were spam or malicious when the company knew they were harmless, for example, the FTC could potentially try to argue that Twitter acted unfairly or deceptively, according to Baer. The FTC has historically enjoyed broad latitude to prosecute alleged unfair and deceptive business practices. And crucially, those cases don’t require a showing of market power. With the FTC already watching Twitter’s behavior closely under Musk, the Mastodon issue may lead to further scrutiny that the company can little afford.