A menorah is installed outside the Tree of Life Synagogue in preparation for a celebration service at sundown on the first night of Hanukkah in December 2018 in the Squirrel Hill neighbourhood of Pittsburgh. Picture: Gene J. Puskar/AP

Pittsburgh - The man accused of massacring 11 people at a Pittsburgh synagogue last year in the deadliest attack on a US Jewish institution pleaded not guilty on Monday in federal court to a dozen additional charges including hate crimes.

The suspect, Robert Bowers, is accused of bursting into the Tree of Life synagogue in the city's Squirrel Hill neighbourhood on October 27 armed with three handguns and a semi-automatic rifle, and firing on congregants as he shouted: "All Jews must die."

Bowers, 46, pleaded not guilty to 19 fresh charges, which bring the total number of criminal counts he is facing to 63, including the use of a firearm to commit murder and obstruction of religious exercise resulting in death.

Federal prosecutor Tony Rivetti said it will take up to three weeks to determine if the government will seek the death penalty.

This Pennsylvania Department of Transportation photo shows Robert Bowers, a truck driver accused of killing 11 and wounding seven during an attack on a Pittsburgh synagogue in October 2018. Picture: Pennsylvania Department of Transportation via AP

One of Bowers' attorneys, Judith Clarke, a death penalty specialist, said the defence hoped to settle without a trial. A negotiated plea deal could allow Bowers to avoid facing the risk of execution.

Bowers, who had frequently posted anti-Semitic slurs and conspiracy theories online, sat quietly through the hearing, dressed in a short-sleeved red jumpsuit and shackled at the ankles. He replied "yes" when U.S. District Judge Donetta Ambrose asked if he understood the charges, and he said he wanted to be tried by a jury.

A member of the congregation in attendance at the hearing, Jon Pushinsky, said "We are here to bear witness. We are here to show that this act of violence will not deter us from serving our community."

Rabbi Eli Wilansky lights a candle after a mass shooting at Tree of Life Synagogue in Pittsburgh's Squirrel Hill neighbourhood on October 27, 2018. File picture: Steph Chamber/Pittsburgh Post-Gazette via AP

The victims of the attack included a 97-year-old woman, two brothers in their 50s and a married couple in their 80s. Two civilians and five police officers were wounded before the suspect was shot by police and surrendered.

On the day of the attack, Bowers, a Pittsburgh resident, said online, "I can't sit by and watch my people get slaughtered. Screw your optics, I'm going in."

The attack followed a wave of politically motivated pipe bombs mailed to prominent Democrats and heightened national tensions ahead of November's midterm congressional elections.

A menorah is tested outside the Tree of Life Synagogue in preparation for a celebration service at sundown on the first night of Hanukkah on Sunday, December 2, 2018, in the Squirrel Hill neighbourhood of Pittsburgh. Picture: Gene J. Puskar/AP

It also fueled a national debate over the rhetoric of US President Donald Trump, who critics say contributed to a surge in white nationalist and neo-Nazi activity. The Trump administration has rejected the notion he has encouraged far-right extremists who have embraced him. 

Reuters