The Twitter accounts of high profile figures and corporations, including former United States President Barack Obama, Democratic presidential candidate Joe Biden, Tesla Chief Executive Officer Elon Musk, Microsoft co-founder Bill Gates and Apple, were among those hacked on Wednesday by scammers trying to dupe people into sending cryptocurrency bitcoin, AFP reported.

Jack Dorsey, Twitter’s chief executive, said in a post that it was a tough day for them. “We all feel terrible this happened,” he tweeted. “We’re diagnosing and will share everything we can when we have a more complete understanding of exactly what happened.”

Twitter said it was aware of “a security incident” and “taking steps to fix it”, but provided no more information. The hack unfolded over the course of several hours and the company disabled the ability to tweet from validated accounts, those with the official blue check mark. “You may be unable to tweet or reset your password while we review and address this incident,” Twitter’s support team said in a post.

The posts, which appeared across multiple accounts in a matter of minutes, promised people they had 30 minutes to send $1,000 in bitcoin in order to be sent back twice as much in return. Some of the tweets were deleted, but re-posted again as Twitter scrambled to contain the incident.

According to Blockchain.com, which monitors transactions made in cryptocurrencies, a total of 12.58 bitcoins, worth over $1,00,000, had been sent to the email addresses mentioned in the fraudulent tweets.

Other targeted accounts include those of rapper Kanye West, reality television show star Kim Kardashian, businessman Michael Bloomberg, Uber, and a number of cryptocurrency exchanges.

Shares in the social media company tumbled almost 5% in after hours trading. The motives and source of the attack are not yet known, but the unusual targeting suggests hackers may have gained access at the system level, rather than through individual accounts. Experts were surprised at the sheer scale and coordination of Wednesday’s incident.

“This appears to be the worst hack of a major social media platform yet,” said Dmitri Alperovitch, who co-founded cybersecurity company CrowdStrike, told Reuters.