(CNN) Saturday's shooting in El Paso is at least the third atrocity this year where a suspect is believed to have posted to 8chan in advance of an attack.

8chan is an online messaging board that has been used by anonymous accounts to share extremist messages and cheer on mass shooters. It is rife with racist and anti-Semitic conspiracy theories.

Law enforcement officials in El Paso, Texas, confirmed on Saturday they were investigating a document posted on 8chan that they believe was written by Patrick Crusius, the shooting suspect.

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In the four-page document on 8chan, the author wrote: "I'm probably going to die today." A CNN analysis found it was posted less than 20 minutes before police received the first calls about the El Paso shooting.

The document is filled with white nationalist and racist hatred toward immigrants and Hispanics, blaming immigrants and first-generation Americans for taking away jobs and the blending of cultures in the US.

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