U.S. officials have not determined who was behind an apparent cyberattack on the social media site X that limited access to the platform for thousands of users.

Elon Musk’s X has been hit by three waves of outages since this morning, which the billionaire said was due to a cyberattack.

Isik Mater, the director of research at NetBlocks, a company that tracks global internet connectivity, told NBC News that X had suffered intermittent outages since Monday morning. While establishing a DDoS attack with certainty can be difficult, Mater said, Musk’s claim was plausible.