The co-founder and chief executive of Twitter has had his own account on the service taken over by hackers.
A group referring to itself as the Chuckling Squad said it was behind the breach of C’s account.
A spokeswoman for Twitter told the BBC that the site was urgently investigating.
The account tweeted out a flurry of highly offensive and racist remarks. The offending tweets appear to have been mostly removed.
The messages – some posted directly by the @jack account, and others retweeted from other accounts – used the n-word and made anti-Semitic comments referencing the Holocaust.
A chat channel on Discord, a separate website, was apparently set up by the group to discuss and joke about the attack – but was quickly shut down.
The Chuckling Squad has taken credit for a number of attacks on high-profile Twitter accounts recently, including @Etika, an account belonging to YouTube personality Desmond Amofah, who died earlier this year in an apparent suicide.
It is as yet unclear how the attackers gained access, though it appears a vulnerability in a third party app could potentially have been to blame.