Twitter says it’s investigating a “coordinated social engineering attack” that saw the accounts of prominent politicians and celebrities briefly hacked on Wednesday. The accounts of former President Barack Obama, Joe Biden, and billionaires including Jeff Bezos, Elon Musk and Bill Gates tweeted out messages linked to a cryptocurrency scam. The hack raised concerns over Twitter’s ability to prevent international incidents and domestic crises, as political leaders, including Donald Trump, use the forum to make policy announcements.