By Joseph Menn and Mark Hosenball

Earlier the law enforcement agency had said hackers appeared to commit cryptocurrency fraud after they seized control of the Twitter accounts of celebrities and political figures including Joe Biden, Kim Kardashian, Barack Obama and Elon Musk.

The Federal Bureau of Investigation did not immediately respond to a question about it leading an inquiry into Twitter.

A day after the breach, it was not clear if the hackers were able to see private messages sent by the account holders. U.S. lawmakers worried about future attacks.

“While this scheme appears financially motivated…imagine if these bad actors had a different intent to use powerful voices to spread disinformation to potentially interfere with our elections, disrupt the stock market, or upset our international relations,” U.S. Senator Ed Markey, a Democrat, said in a statement.

Echoing a similar sentiment, Representative Jim Jordan, the top Republican on the House Judiciary Committee, asked what would happen if Twitter allowed a similar incident to occur on Nov. 2, a day before the U.S. presidential election.

Jordan said he remained locked out of his Twitter account as of Thursday morning and said his confidence in how the company operates has been deteriorating.

Twitter Inc said hackers had targeted employees with access to its internal systems and “used this access to take control of many highly-visible (including verified) accounts and Tweet on their behalf”.

Other high-profile accounts that were hacked included rapper Kanye West, Amazon.com Inc founder Jeff Bezos, investor Warren Buffett, Microsoft Corp co-founder Bill Gates, and the corporate accounts for Uber Technologies Inc and Apple Inc.

In an extraordinary step, it temporarily prevented many verified accounts from publishing messages as it investigated the breach.

The hijacked accounts tweeted out messages telling users to send bitcoin and their money would be doubled. Publicly available blockchain records show that the apparent scammers received more than $100,000 worth of cryptocurrency.

Twitter’s shares fell a little over 1% on Thursday afternoon.

CEO Jack Dorsey said in a tweet on Wednesday that it was a “tough day” for everyone at Twitter and pledged to share “everything we can when we have a more complete understanding of exactly what happened”.

Dorsey’s assurances did not assuage Washington’s concerns about social media companies, whose policies have come under scrutiny by critics on both the left and the right.

Democratic Senator Mark Warner called on Twitter and law enforcement to investigate the matter while the U.S. House Intelligence Committee said it was in touch with Twitter regarding the hack, according to a committee official who did not wish to be named.

Republican Senator Josh Hawley wrote a letter to Dorsey within minutes of the hack and asked about potential data theft and whether the breach affected select users or the security of the platform overall.

Frank Pallone, a Democrat who chairs the House Energy and Commerce Committee that oversees a sizeable portion of U.S. tech policy, said in a tweet the company “needs to explain how all of these prominent accounts were hacked.”

The New York State Department of Financial Services also weighed in, saying it will investigate the hack.

(Reporting by Joseph Menn and Mark Hosenball; Writing by Nandita Bose; Additional reporting by Ayanti Bera, Aakash Jagadeesh Babu and Subrat Patnaik in Bengaluru and Nandita Bose, David Shepardson and Diane Bartz in Washington Editing by Peter Graff, Carmel Crimmins and Lisa Shumaker)