Twitter briefly suspended David Duke, a former Ku Klux Klan leader, on Monday. 

For a few hours Monday morning, users who tried viewing Duke's tweets saw a message that the account was suspended.  The company reinstated the account, and Duke tweeted that he didn't know why he had been suspended. 

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In recent months, Twitter has doubled down on policing abuse, harassment, and hate speech on its platform. 

Last week, the company added an automated system for banning users who violate its terms of service, and implemented new tools for individuals to be able to mute accounts. 

Twitter previously banned white nationalist Richard Spencer, though his account was reinstated.  Last year, the social network permanently banned Breitbart editor Milo Yiannopoulos after he instigated a torrent of harassment at actress Leslie Jones. 

Critics say Twitter isn't doing enough to prevent harassment and bullying on its platform, and the company is working to change that.  For instance, during the 2016 presidential election, anti-Semitic attacks on Twitter users increased.  In October, the Anti-Defamation League found almost 20,000 of those tweets targeted 800 journalists. 

It's unclear what prompted the social network to shut down Duke's account, but he has previously tweeted anti-Semitic and racist comments.  Prior to the suspension, he had over 30,000 followers.  After being reinstated, he had just over 100 followers.  It took several hours for the follower count to reach the original level. 

Twitter did not immediately respond to a request for comment. 

"I can make no sense why people lose their accounts and get them reinstated," Heidi Beirich, director of Southern Poverty Law Center's Intelligence Project, said. " I feel like if Twitter wants to get serious about reigning in hate speech, there's no reason for a David Duke or a Richard Spencer to be on the platform." 

The SPLC tracks hate speech on Twitter, and when Twitter banned some white nationalist accounts in November, the organization highlighted numerous examples of extremist accounts that were still active on the site.