NEW YORK (AP) — Twitter shares fell Thursday after President Donald Trump said in a tweet the company was limiting visibility of prominent Republicans and said he was going to look into the matter.

The tweet came after a story in Vice magazine said prominent Republicans such as Republican Party chair Ronna McDaniel were not showing up in one aspect of Twitter’s search function.

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While the accounts did show up during a general search, they didn’t automatically populate in a drop-down bar when typing in the name.

In his tweet Trump accused Twitter of “shadow banning,” or limiting visibility, of some Republicans and said it was “discriminatory and illegal.”

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Twitter said it does not “shadow ban.” The company said it was aware that some accounts were not automatically populating but that was due to a bug they are working to fix. The social media site said in a statement it does not make judgments based on political views or substance of tweets.

Social media sites like Facebook and Twitter have been accused of having a bias against conservatives before. Diamond and Silk, the Trump-supporting video personalities, had their page removed by Facebook earlier this year, leading to outcries of censorship, including from several congressmen. During his Congressional hearing in April, Facebook founder Mark Zuckerberg said this was an “enforcement error” and the page would be reinstated.

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Twitter shares fell 4 percent in afternoon trading.