A mass murderer who is accused of murdering 11 people at the Tree of Life synagogue ranted incessantly on social media about his hatred of Jewish people, a court has heard. 

New evidence was introduced at his Robert Bowers' federal death penalty trial on Tuesday. 

Prosecutors are trying to show Bowers was motivated by extreme hostility toward Jews when he opened fire at the Tree of Life synagogue in October 2018 during Sabbath services.

Bowers could face the death penalty if convicted of some of the 63 counts he faced for the October 2018 attack. It is considered the deadliest antisemitic attack in US history.

With the federal trial in its third week, testimony turned toward Bowers' use of Gab - a social media platform popular with the far right - to advance his antisemitic views.

According to Associated Press, Bowers' Gab profile said ‘Jews are the children of Satan’, and he posted, liked or shared a stream of virulently antisemitic content, according to dozens of examples provided to the jury.

Bowers also called Jews ‘public enemy number one,’ according to a post read by an FBI agent, spoke approvingly of Adolf Hitler and the Nazi extermination of Jews, and shared an image that said "the only good Jew is a dead Jew."

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Mourners honour the victims outside the Tree of Life Synagogue (Credit: Getty Images)

William Braniff, a terrorism expert at the US Department of Homeland Security who reviewed Bowers' social media activity, told a Pittsburgh court that Bowers used symbols and terms associated with Nazi, antisemitic and white supremacist ideologies.

Meanwhile, Bowers posted about HIAS, a Jewish agency that helps refugees resettle in the United States on the morning of the attack. 

Dor Hadash, one of the three congregations that shared the Tree of Life building, worked with HIAS.

Bowers posted: "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.”

His lawyers, who are pushing for a custodial sentence instead of the death penalty,  acknowledged he was the shooter but are trying to raise questions about motive. 

The defence suggested he did not act out of religious hatred but rather a delusional belief that Jews were enabling genocide by helping immigrants come to the US.

The trial continues.