Eleven people are dead after a shooter opened fire at a Pennsylvania synagogue during a service on Saturday.

The alleged gunman has been identified by authorities as 46-year-old Robert Bowers. He allegedly opened fire at the Tree of Life Congregation in Pittsburgh after screaming, "All Jews must die," KDKA reported, citing police.

"It's a very horrific crime scene," Pittsburgh Public Safety Director Wendell D. Hissrich said at a press conference. "It's one of the worst I've seen."

At least six others were injured, four of whom were police officers who raced to the scene, according to authorities.

The officers’ injuries are not believed to be life-threatening, according to reports, but the other two victims were listed in critical and serious condition. No children were injured, Hissrich said.

Police were called to the scene by several people inside of the building just before 10 a.m. The suspect and police exchanged gunfire before he surrendered, authorities said.

The gunman was taken to hospital with multiple gunshot wounds and is in fair condition, the Pittsburgh Post-Gazette reported.

Authorities said the shooting is being investigated as a federal hate crime.

Jonathan Greenblatt, CEO of the Anti-Defamation League, said the group believes it is the deadliest attack on the Jewish community in American history.

‘‘Our hearts break for the families of those killed and injured at the Tree of Life Synagogue, and for the entire Jewish community of Pittsburgh,’’ Greenblatt told the Associated Press.

President Trump condemned the attacks during a speech at an event in Indiana Saturday.

"This wicked act of mass murder is pure evil, hard to believe and frankly something that is unimaginable, our nation and the world are shocked," Trump said. He added that "vile, hate-filled poison of anti-Semitism" needs to be rejected.

Governor Tom Wolf also issued a statement after the tragedy.

“The shooting in Pittsburgh this morning is an absolute tragedy," Wolf said. “These senseless acts of violence are not who we are as Americans. My thoughts right now are focused on the victims, their families and making sure law enforcement has every resource they need."

A Gofundme page has been established for the victims.

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