Federal prosecutors in Pittsburgh said Monday that they intend to seek the death penalty against the man accused of gunning down 11 people at a synagogue last year.

Robert Bowers, who faces a 63-count indictment, has pleaded not guilty to hate crimes and other offenses.

Authorities have said Bowers was armed with multiple weapons, including a Colt AR-15 rifle and three handguns, when he opened fire inside the Tree of Life synagogue in the city's Squirrel Hill neighborhood on Oct. 27 during a morning worship service.

The attack left 11 people dead and several others injured, including police officers who were trying to rescue victims and confront the gunman. Worshipers told MSNBC at the time that a circumcision celebration, known as a bris, was taking place when the first shots rang out.

Bowers, a truck driver, allegedly talked about his desire to "kill Jews" amid the rampage. He had a history of threatening Jewish communities and posting anti-Semitic conspiracy theories on the internet. In the hours before the attack, for example, Bowers wrote that the Hebrew Immigrant Aid Society, a nonprofit humanitarian group, had brought immigrants into the U.S. to commit violence.

The post said: "Screw your optics, I'm going in."

Prosecutors had previously said they would pursue the death penalty against Bowers. But on Monday, the U.S. attorney's office in Pittsburgh filed a notice of intent, formalizing their plans.

The filing said justification for a death sentence includes allegations of substantial planning and premeditation, the vulnerability and number of victims, and a motivation of religious hostility, according to the Associated Press.