Tipsters alerted the FBI to a threatening post on the anonymous message board 8chan minutes before a gunman opened fire at a California synagogue, an FBI official told CNN Monday.

The post was written by someone using the same name as the 19-year-old who would later be arrested in the shooting at the Congregation Chabad in Poway, north of San Diego.

The FBI official said several tips came in to its tip website and phone line approximately five minutes before John T. Earnest allegedly began shooting. He opened fire at 11:23 a.m. local time, according to the San Diego Sheriff’s office.

The FBI was not able to determine the author of the 8chan post or the location of their intended attack before the shooting.

The 8chan post linked to a hate speech-laden “open letter” signed with the name John Earnest. The letter talks about killing Jewish people without making reference to Poway, San Diego or Congregation Chabad.

On its Twitter account, 8chan claims they deleted the post nine minutes after it was published. CNN has reached out to 8chan and asked if they alerted law enforcement to the post, but has not yet received a response.

The shooting in Poway left one person dead and injured three others, including Rabbi Yisroel Goldstein, the leader of the synagogue.