Twitter acknowledged Wednesday evening that a third party was able to target its employees with "social engineering" techniques to gain control of the accounts of some of its most prominent users, uncorking a Bitcoin scam and sending the internet into a tizzy for several hours.

"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 through its Twitter Support account hours after the first hijacked tweets were sent. "We know they used this access to take control of many highly visible (including verified) accounts and Tweet on their behalf."

Sign up for Protocol newsletters

The accounts of Jeff Bezos, Tim Cook, Bill Gates, Elon Musk, Joe Biden, Kanye West, Kim Kardashian-West and a number of other major figures in tech, media and politics began tweeting out the same message midafternoon on the West Coast, urging their combined tens of millions followers to donate thousands of dollars in Bitcoin to a mystery account.

Shortly thereafter, verified users lost the ability to post. Unverified users began celebrating with tweets and GIFs. "The revolution will be unverified," one Twitter user wrote.

The block on verified accounts prevented major news organizations, federal agencies, police departments and state governments from tweeting out information.

Most of the messages, which began with the phrase "I am giving back to the community," promised to double the donations of anyone who sent Bitcoin to a particular wallet. Within hours, more than 300 people had sent more than $110,000.

More than an hour after the hacks began, scam tweets were still being sent from the accounts of additional celebrities. "We have locked accounts that were compromised and will restore access to the original account owner only when we are certain we can do so securely," Twitter Support said late Wednesday.

A key question Twitter will need to answer in the coming days and weeks is not just how this breach occurred, but also how much access the hackers had to those users' accounts. The collateral damage could be infinitely larger, for instance, if the hackers had a window into these people's direct messages.

"Internally, we've taken significant steps to limit access to internal systems and tools while our investigation is ongoing. More updates to come as our investigation continues," Twitter Support said late Wednesday.

The Biden campaign told Protocol in a statement, "Twitter locked down the account immediately following the breach and removed the related tweet. We remain in touch with Twitter on the matter." The campaign didn't immediately respond to a question about whether it had two-factor authentication enabled.

Shortly after the breach, Republican Sen. Josh Hawley of Missouri sent a letter to Twitter CEO Jack Dorsey demanding answers about the scope of the damage and asking Dorsey to contact the Department of Justice and the Federal Bureau of Investigations for help looking into the matter. "I am concerned that this event may represent not merely a coordinated set of separate hacking incidents but rather a successful attack on the security of Twitter itself," the letter reads. "A successful attack on your system's servers represents a threat to all of your users' privacy and data security."

Twitter CEO Jack Dorsey addressed the hack in a tweet earlier Wednesday evening, but it didn't explain much. "Tough day for us at Twitter," Dorsey wrote. "We all feel terrible this happened. We're diagnosing and will share everything we can when we have a more complete understanding of exactly what happened." He ended with a blue heart emoji for "our teammates working hard to make this right."

Update: This post was updated at 8:15 p.m. PT with more information from Twitter Support.