The hijacking of several prominent Twitter accounts, including that of presumptive Democratic U.S. presidential nominee Joe Biden, has again raised questions about the company’s ability to combat disinformation on its platform and rekindled concerns about potential election interference with the November U.S. election just four months away.

Twitter Inc. and other social media sites such as Facebook Inc. were already facing scrutiny over their ability and willingness to protect the integrity of the democratic process and avoid a repeat of 2016, when Russia spread disinformation in efforts to bolster Donald Trump’s candidacy.

That pressure increased on Wednesday as Biden, Barack Obama, Jeff Bezos, Bill Gates, Warren Buffett and Elon Musk were among the prominent political and business leaders whose accounts were exploited to send out tweets in an apparently coordinated effort to promote a cryptocurrency scam.