Billionaires Elon Musk, Jeff Bezos and Bill Gates are among many prominent US figures targeted by hackers on Twitter in an apparent Bitcoin scam.

A series of high-profile Twitter accounts were hijacked on Wednesday, with some of the platform`s top voices – including U.S. presidential candidate Joe Biden, former U.S. President Barack Obama, billionaire Elon Musk, reality television show star Kim Kardashian and rapper Kanye West, among many others – used to solicit digital currency.

Nearly two hours after the first wave of hacks, the cause of the breach had not yet been made public. In a sign of the seriousness of the problem, Twitter took the extraordinary step of preventing at least some verified accounts from publishing messages altogether.

It was not clear whether all verified users were affected but, if they were, it would have a huge impact on the platform and its users. Verified users include celebrities, journalists, and news agencies as well as governments, politicians, heads of state, and emergency services.

The unusual scope of the problem suggests hackers may have gained access at the system level, rather than through individual accounts. While account compromises are not rare, experts were surprised at the sheer scale and coordination of Wednesday`s incident.