It was about 4 in the afternoon Wednesday on the East Coast when chaos struck online. Dozens of the biggest names in America — including Joe Biden, Barack Obama, Kanye West, Bill Gates and Elon Musk — posted similar messages on Twitter: Send Bitcoin and the famous people would send back double your money.

It was all a scam, of course, the result of one of the most brazen online attacks in memory.

A first wave of attacks hit the Twitter accounts of prominent cryptocurrency leaders and companies. But soon after, the list of victims broadened to include a Who’s Who of Americans in politics, entertainment and tech, in a major show of force by the hackers.

The Associated Press reported that Amazon CEO Jeff Bezos was also among those whose Twitter accounts were hacked.

Twitter quickly removed many of the messages, but in some cases similar tweets were sent again from the same accounts, suggesting Twitter was powerless to take back control of the accounts.

The company eventually disabled broad swaths of its service, including the ability of verified users to tweet, as it scrambled to prevent the scam from spreading further. The company sent a tweet saying it was investigating the problem and looking for a fix. “You may be unable to Tweet or reset your password while we review and address this incident,” the company said in a second tweet.

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The hackers did not use their access to take aim at any important institutions or infrastructure — instead just asking for Bitcoin. But the attack was concerning to security experts because it suggested the hackers could have easily caused much more havoc.

It was the nature of the attack — “effective, but also amateurish” in the words of one senior American intelligence official — that led U.S. intelligence agencies to an initial assessment this was most likely the work of an individual hacker, not a state. Had it been Russia, China, North Korea or Iran, said the official, who would not speak on the record because of not being authorized to discuss an intelligence investigation, the effort would have probably focused on trying to trigger stock market havoc or perhaps the issuance of political pronouncements in the name of Biden or other targets.

Officials also noted the breach did not affect the account of one of the most watched and powerful users of Twitter: President Donald Trump. Trump’s account is under a special kind of lock and key after past incidents, the official noted.

Security experts said the wide-ranging attacks hinted the problem was caused by a security flaw in Twitter’s service, not by lax security measures used by the people who were targeted.

Alex Stamos, director of the Stanford Internet Observatory and the former chief security officer at Facebook, said one of the leading theories among researchers was that the hacker, or hackers, had obtained the encryption keys to the system, which enabled them to essentially imitate or steal the “tokens” that grant access to individual accounts.

There were a range of other theories, he said, but all suggested the attackers got inside Twitter’s system, rather than stealing the passwords of individual users. One American official called that a “scary possibility” in a world where national leaders, sometimes imitating Trump’s techniques, have adopted Twitter as a primary source of unfiltered communications.

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“It could have been much worse. We got lucky that this is what they decided to do with their power,” Stamos said.

The hacker or hackers made some rookie errors. Stamos said because the attackers had sent identical messages from the compromised accounts, they were easy to detect and delete. The decision to ask for money through Bitcoin, he added, showed the attackers were most likely unable or unwilling to launder money or use their access for a more sophisticated scam.

The messages were a version of a long-running scam in which hackers pose as public figures on Twitter, and promise to match or even triple any funds that are sent to their Bitcoin wallets. But the attacks Wednesday were the first time the real accounts of public figures were used in such a scam.

Bitcoin is a popular vehicle for this type of scam because once a victim sends money, the design of Bitcoin, with no institution in charge, makes it essentially impossible to recover the money.

By Wednesday evening, the Bitcoin wallets promoted in the tweets had received more than 300 transactions and held Bitcoin worth more than $100,000, according to websites that track Bitcoin’s public ledger of transactions, which is known as the blockchain.

Twitter initially handled the attacks by taking down the offending tweets. A spokesman for the Biden campaign said Twitter had removed the tweet promoting the scam and locked down Biden’s account.

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But the hackers kept control of many of the accounts, such as those of Musk and West, and sent out new messages as soon as the old ones were taken down.

Twitter has fallen victim to breaches before.

In August, hackers compromised the account of Twitter’s chief executive, Jack Dorsey, and posted racist messages and bomb threats. Dorsey’s account was taken over after hackers transferred his phone number to a new SIM card, which stores a phone’s number. The practice, known as SIM-swapping, allowed hackers to tweet from Dorsey’s account.

In 2017, a rogue worker at the company used their access to Twitter’s systems to briefly delete Trump’s Twitter account. The account was restored within minutes, but the incident raised questions about Twitter’s security as it serves as a megaphone for politicians and celebrities.

And in 2010, Twitter settled a complaint brought by the Federal Trade Commission, in which the regulator claimed the company did not do enough to protect users’ personal information. The FTC charged that “serious lapses” in Twitter’s security allowed hackers to take control of company systems and send out phony tweets from high-profile accounts, including Obama’s. As part of the settlement, Twitter agreed to undergo security audits for 10 years.

On Wednesday evening, Sen. Josh Hawley, R-Mo., wrote a letter to Dorsey asking for information on the attack, including how many users were compromised.

Shares in the social-media company fell 3% in after-hours trading.

Cybersecurity experts said the attack showed how vulnerable social media remains to attacks.

“This demonstrates a real risk for the elections,” Stamos said. “Twitter has become the most important platform when it comes to discussion among political elites, and it has real vulnerabilities.”