Twitter is investigating a massive hack in which high-profile users from Elon Musk to Joe Biden had their accounts hijacked by scammers, who the social network believes targeted its employees to gain access to internal systems. Posts trying to dupe people into sending hackers the virtual currency Bitcoin were tweeted by the official accounts of Apple, Uber, Kanye West, Bill Gates, Barack Obama and many others on Wednesday. "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. "They used this access to take control of many highly-visible... accounts," the company said, adding that it was investigating "what other malicious activity they may have conducted or information they may have accessed." The fraudulent posts, which were largely deleted, said people had 30 minutes to send $1,000 in the cryptocurrency, promising they would receive twice as much in return. A total of 12.58 bitcoins -- worth almost $116,000 -- were sent to email addresses mentioned in the tweets, according to the site Blockchain.com, which monitors crypto transactions. "Tough day for us at Twitter," chief executive Jack Dorsey said in a tweet. "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." - Blue checkmarks - Twitter said it had locked down the affected accounts and removed the tweets posted by the hackers. "Most accounts should be able to Tweet again," the Twitter support team said in an evening update, having earlier temporarily disabled all posts from verified accounts with an official blue checkmark. But the firm told users that it "may take further actions and will update you if we do." US President Donald Trump's account, which has more than 83 million followers, was not among those targeted, but many specialist Bitcoin firms were. "All major crypto Twitter accounts have been compromised," Cameron Winklevoss, co-founder of the Gemini cryptocurrency exchange, said in a tweet on Wednesday. "This is a SCAM, DO NOT participate!" he warned. Vice reported that a Twitter insider was responsible, citing leaked screenshots and two anonymous sources apparently behind the hack, one of whom told the media outlet they had paid the employee. US Senator Josh Hawley tweeted a letter to Dorsey expressing concern over privacy for the San Francisco-based company's millions of users worldwide. "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," he said. BitTorrent chief executive Justin Sun was offering a $1 million reward for bringing the Twitter hackers to justice, reports said. -'Giving back'- The tweet that appeared on Tesla founder Musk's Twitter feed said: "Happy Wednesday! I am giving back Bitcoin to all of my followers. I am doubling all payments sent to the Bitcoin address below. You send 0.1 BTC, I send 0.2 BTC back!" It added that the offer was "only going on for 30 minutes." The fake messages that appeared on other famous accounts made similar promises of instant riches. One version of the scam invited people to click on a link at which they would be exploited. The BBC reported that a website address in some of the duplicitous tweets had been registered under the name "Anthony Elias", which appeared to be a play on the words "An alias". Twitter has been targeted by hackers in the past. In March 2017, the accounts of Amnesty International, the French economics ministry and the BBC's North America service were broken into by hackers believed to have been loyal to Turkish President Recep Tayyip Erdogan. Last August, a series of insulting or racist messages were posted on the personal account of Twitter founder Dorsey without his knowledge.