President Donald Trump’s Twitter account is once again the subject of debate, as the Knight First Amendment Institute urged him to unblock individuals in a letter sent to the White House on Tuesday.

The institute, a non-profit organization affiliated with Columbia University, said it believes his account is a “designated public forum” and threatened legal action if the president didn’t comply. In a letter addressed to Trump, his counsel, press secretary and social media director, the institute wrote on behalf of Holly O’Reilly and Joseph M. Papp, both of whom criticized the president on the social media platform in recent weeks and said they were blocked.

“The blocking of users from your Twitter account suppresses speech in a number of ways,” the letter stated. “Users who have been blocked cannot follow you on Twitter, and they are limited in their ability to view your tweets, find your tweets using Twitter’s search function, and learn which accounts follow you. They are also limited in their ability to participate in comment threads associated with your tweets.” Replying to messages posted by Trump on Twitter, often in the form of a thread, is common practice on the platform, both by fans and detractors of the Republican president.