Twitter is investigating a massive hack in which high-profile users from Elon Musk to Joe Biden had their accounts hijacked by scammers who the social network believes targeted its employees to gain access to internal systems.

Posts trying to dupe people into sending hackers the cryptocurrency Bitcoin were tweeted by the official accounts of Apple, Uber, Kanye West, Bill Gates, Barack Obama and many others on Wednesday.

“We detected what we believe to be a coordinated social engineering attack by people who successfully targeted some of our employees with access to internal systems and tools,” Twitter said.