The man suspected of storming a Pittsburgh synagogue said a Jewish refugee charity brought "invaders in that kill our people" into the US.
Robert Bowers has been charged with 29 counts of criminal activity, including hate crimes and using a firearm to commit murder, after the attack at the Tree of Life synagogue during a baby naming ceremony.
Eleven people died after he opened fire with what appeared to be an assault rifle and three handguns and is believed to have acted alone, according to authorities.
Just hours before the attack, a social media post on a site called Gab, apparently by Bowers said a Jewish refugee organisation called the Hebrew Immigrant Aid Society (HIAS) "likes to bring invaders in that kill our people".
"I can't sit by and watch my people get slaughtered. Screw your optics, I'm going in."
According to the AP news agency, the cover photo for the account featured a neo-Nazi symbol.
Recent posts on the account reportedly included a photo of a fiery oven like those used in Nazi concentration camps to cremate Jewish people during World War Two.
Other posts are believed to have referenced false conspiracy theories suggesting the Holocaust, in which an estimated six million Jews were killed, was a hoax.
In another post, President Donald Trump was criticised for allowing Jewish people to enter the US.
Gab, which has condemned the synagogue attack, is a Philadelphia-based social networking service created as an alternative to Twitter.
In a statement, Gab.com confirmed the profile belonged to Bowers and also said it had suspended the account and contacted the FBI about it. Gab is popular with far-right extremists.
HIAS is a non-profit group that helps refugees around the world find safety and freedom. The organisation said it is guided by Jewish values and history.
President and chief executive Mark Hetfield said he was not aware of the alleged gunman's "obsession with HIAS until this morning".
A neighbour of the suspect described him as an "average 50-year-old dude".
Chris Hall said of Bowers: "The most terrifying thing is just how normal he seemed.
"He kept to himself. He would a smoke a cigarette in his car; go for a drive and then be back at odd hours, TV would be on, but I couldn't really hear."
Mr Hall said he had never received any antisemitic flyers from Bowers, or spotted any strange tattoos on him, and never saw anyone who may have been Bowers' partner.
He was not known to law enforcement officials before Saturday's attack, said the FBI.
The bureau said Bowers, who is believed to have shouted antisemitic slurs during the attack, murdered worshippers and was then leaving the building when he came into contact with a uniformed police officer.
The pair exchanged gunfire and Bowers then went back inside before a SWAT team arrived, according to the FBI.
After a shootout, he surrendered.
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He was taken to hospital with multiple gunshot wounds, which were not believed to be self-inflicted.
He is described as being in a fair condition.