Joe Biden, Elon Musk, Jeff Bezos and other high-profile Twitter account holders were the targets of a widespread hack to offer fake bitcoin deals on Wednesday in one of the most pronounced security breaches on a social media site.

Accounts for former president Barack Obama, Microsoft co-founder Bill Gates, musician Kanye West and both Uber and Apple also posted similar tweets, all instructing people to send cryptocurrency to the same bitcoin address. The tweets were removed throughout the afternoon, shortly after being posted.

There have been hacks of high-profile individual accounts on Twitter before, including Twitter chief executive Jack Dorsey last year. But the widespread nature of this attack suggested an unusually broad access to internal controls. While it was unclear how the attacks originated or why they went on for hours, some cybersecurity experts speculated that someone may have gained access to internal Twitter controls that allowed them to take over and post on the accounts.

“This is massive,” said cybersecurity expert Rachel Tobac, the CEO of SocialProof Security. “This is most likely the largest attack I’ve ever seen. We are extremely lucky that these attackers are monetarily motivated and not sowing mass chaos all over the world.”

The attack also partially shut down the network. Twitter said in a tweet on Wednesday afternoon that some users weren’t able to tweet while it was addressing the incident. Users with the check mark indicating that their accounts were verified by Twitter reported that they weren’t able to tweet.

Twitter started letting verified accounts tweet again Wednesday night but warned the “functionality may come and go” as it worked on a fix to the breach. Later the same night, Dorsey tweeted that the company was “diagnosing and will share everything we can when we have a more complete understanding of exactly what happened.” He called it a “tough day” at Twitter.

Twitter said in a later tweet that it “detected a coordinated attack by people who successfully targeted some of our employees with access to internal systems and tools.” The hackers used that access to take over the accounts.

The breach will create major optics challenges for Twitter, and it will make it more challenging moving forward to verify the authenticity of messages on the service, cybersecurity experts warned. That could have wide-reaching implications for politicians, celebrities and brands that use Twitter as an essential channel for communication.

“The problem is that we all rely on Twitter as this public space that is safe and secure, and we know that the tweets that someone like a Joe Biden is sending out are authentic,” said Harper Reed, an entrepreneur who served as the 2012 Obama campaign’s chief technology officer. “Twitter has proven to us that may not be true.”

Joe Biden, Elon Musk, Jeff Bezos and other high-profile Twitter account holders were the targets of a widespread hack to offer fake bitcoin deals on Wednesday in one of the most pronounced security breaches on a social media site.

Accounts for former president Barack Obama, Microsoft co-founder Bill Gates, musician Kanye West and both Uber and Apple also posted similar tweets, all instructing people to send cryptocurrency to the same bitcoin address. The tweets were removed throughout the afternoon, shortly after being posted.

There have been hacks of high-profile individual accounts on Twitter before, including Twitter chief executive Jack Dorsey last year. But the widespread nature of this attack suggested an unusually broad access to internal controls. While it was unclear how the attacks originated or why they went on for hours, some cybersecurity experts speculated that someone may have gained access to internal Twitter controls that allowed them to take over and post on the accounts.

“This is massive,” said cybersecurity expert Rachel Tobac, the CEO of SocialProof Security. “This is most likely the largest attack I’ve ever seen. We are extremely lucky that these attackers are monetarily motivated and not sowing mass chaos all over the world.”

The attack also partially shut down the network. Twitter said in a tweet on Wednesday afternoon that some users weren’t able to tweet while it was addressing the incident. Users with the check mark indicating that their accounts were verified by Twitter reported that they weren’t able to tweet.

Twitter started letting verified accounts tweet again Wednesday night but warned the “functionality may come and go” as it worked on a fix to the breach. Later the same night, Dorsey tweeted that the company was “diagnosing and will share everything we can when we have a more complete understanding of exactly what happened.” He called it a “tough day” at Twitter.

Twitter said in a later tweet that it “detected a coordinated attack by people who successfully targeted some of our employees with access to internal systems and tools.” The hackers used that access to take over the accounts.

The breach will create major optics challenges for Twitter, and it will make it more challenging moving forward to verify the authenticity of messages on the service, cybersecurity experts warned. That could have wide-reaching implications for politicians, celebrities and brands that use Twitter as an essential channel for communication.

“The problem is that we all rely on Twitter as this public space that is safe and secure, and we know that the tweets that someone like a Joe Biden is sending out are authentic,” said Harper Reed, an entrepreneur who served as the 2012 Obama campaign’s chief technology officer. “Twitter has proven to us that may not be true.”