The account of Twitter Chief Executive Jack Dorsey was hacked, sending public tweets and retweets including racial slurs and curse words to 4 million followers before Twitter secured the account.

 The offensive tweets and retweets were deleted less than an hour after the incident. The social media company said the phone number associated with his account was compromised due to a security oversight by the mobile provider.

“This allowed an unauthorized person to compose and send tweets via text message from the phone number. That issue is now resolved.” the company said. There was no indication that Twitter’s systems had been compromised, it added.

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One of the tweets from the hacked account said Nazi leader Adolf Hitler was innocent, while others contained derogatory comments about black people and Jews. There was also a tweet suggesting there was a bomb at Twitter’s headquarters.

This was not the first time that Dorsey’s Twitter account has been hacked. His account was compromised in 2016 by a group that also hacked the Twitter accounts of Google CEO Sundar Pichai and Facebook CEO Mark Zuckerberg.

Twitter shares were down less than 1% in after-hours trade following the hack.