"It cannot be overstated how troubling this incident is, both in its effects and in the apparent failure of Twitter’s internal controls to prevent it," Senate Commerce Committee Chair Roger Wicker wrote in one of several missives by members of Congress to Dorsey on Thursday.

Hackers penetrated the accounts of Joe Biden, Bill Gates, Barack Obama, Elon Musk and dozens of other prominent Americans Wednesday afternoon, sending Twitter messages in their names telling followers to send money using Bitcoin and promising to send back twice the amount.

Twitter blocked users from being able to send Bitcoin addresses as it sought to clean up Wednesday’s mess.