(CNN) A Parkland shooting survivor and pro-Second Amendment activist said Harvard University rescinded his acceptance as a result of racist remarks he made before the 2018 massacre at Marjory Stoneman Douglas High School in Florida.

Kyle Kashuv disclosed the rescinding Monday in a Twitter thread, acknowledging that he and classmates, then 16, made "abhorrent racial slurs" in digital messages almost two years ago "in an effort to be as extreme and shocking as possible."

He wrote an apology for his remarks and posted a screenshot of what appears to be a June 3 letter from Harvard Dean of Admissions William Fitzsimmons, rescinding his admission.

1/ THREAD: Harvard rescinded my acceptance.



Three months after being admitted to Harvard Class of 2023, Harvard has decided to rescind my admission over texts and comments made nearly two years ago, months prior to the shooting.



I have some thoughts. Here's what happened. — Kyle Kashuv (@KyleKashuv) June 17, 2019

"Harvard deciding that someone can't grow, especially after a life-altering event like the shooting, is deeply concerning. If any institution should understand growth, it's Harvard, which is looked to as the pinnacle of higher education despite its checkered past," Kashuv said on Twitter.

"Throughout its history, Harvard's faculty has included slave owners, segregationists, bigots and antisemites," he added. "If Harvard is suggesting that growth isn't possible and that our past defines our future, then Harvard is an inherently racist institution. But I don't believe that."

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