SAN FRANCISCO (AP) - Twitter chief executive officer Jack Dorsey's account was hacked, and whoever broke in sent racist and vulgar tweets to his 4.2 million followers.

Some of the tweets were up for about 30 minutes before Twitter took them down.

The tweets included messages such as "Hitler is innocent" and, using a vulgarity, asked "bald skeleton head tramp," apparently referring to Dorsey, to unsuspend certain accounts.

Twitter says it is investigating.

The company suspended accounts that the hacker or hackers retweeted while they had control of Dorsey's account.

Twitter also suspended the account that appeared to be responsible for the hack.

Based on some of the tweets sent from Dorsey's account, a group called Chuckle Squad was likely responsible.

The incident comes as Twitter and Dorsey have promised to improve the "health" and civility of discourse on the social media service, cracking down on hate speech and abuse.

Long criticised for allowing bad behaviour to run rampant, Twitter has been trying to rein in the worst offenders, banning accounts that violate its terms and making others less visible.

While Twitter says it is making progress, it has met with criticism both from those who say it's not doing enough and from others who say it's going too far in limiting speech.

Conservatives also complain of bias.

Last year, the company permanently banned right-wing conspiracy theorist Alex Jones and his Infowars show for abusive behaviour.

This is not the first time Dorsey has been hacked. In 2016, his account was taken over by the hacker group OurMine, which also attacked other high-profile social media accounts.