A survivor of the Pittsburgh synagogue massacre described on Sunday how he and other terrorised worshippers concealed themselves in a supply closet as the gunman stepped over the body of a man he had just shot and killed, entered their darkened hiding spot and looked around.

“I can’t say anything, and I’m barely breathing,” recalled Barry Werber, 76, in an interview with The Associated Press. “He didn’t see us, thank God.”

The gunman, Robert Gregory Bowers, opened fire with an AR-15 rifle and other weapons during worship services inside Tree of Life Synagogue, killing eight men and three women before a tactical police team tracked him down and shot him, according to state and federal affidavits made public on Sunday. He expressed hatred of Jews during the rampage and later told police that “all these Jews need to die”, authorities said.

Six people were injured in the attack, including four officers.

Rabbi Jeffrey Myers (right) of Tree of Life Congregation hugs Rabbi Cheryl Klein (left) of Dor Hadash Congregation and Rabbi Jonathan Perlman during a community gathering held in the aftermath of the deadly shooting, AP

Bowers targeted a building that housed three separate congregations, all of which were conducting Sabbath services when the attack began just before 10 am in the tree-lined residential neighborhood of Squirrel Hill, about 10 minutes from downtown Pittsburgh and the hub of the city’s Jewish community.

As authorities worked to piece together Bowers’s background and movements, harrowing accounts from the survivors began to emerge.

Speaking at a vigil in Pittsburgh on Sunday night, Tree of Life Rabbi Jeffrey Myers said about a dozen people had gathered in the synagogue’s main sanctuary when Bowers walked in and began shooting. He said seven of his congregants were killed.

“My holy place has been defiled,” he said.

Officials released the names of all 11 of the dead, all of them middle-aged or elderly. The victims included intellectually disabled brothers and a husband and wife. The youngest was 54, and the oldest was 97.

“The loss is incalculable,” said Stephen Cohen, co-president of New Light Congregation, which rents space at Tree of Life.

Mayor Bill Peduto called it the “darkest day of Pittsburgh’s history”.

Bowers shot his victims with an AR-15 — the weapon used in many of the nation’s mass shootings — and three handguns, all of which he owned legally and had a license to carry, according to a law enforcement official who wasn’t authorised to discuss the ongoing investigation, and who spoke on Sunday on condition of anonymity.

US attorney Scott Brady said on Sunday night in Pittsburgh that federal prosecutors intend to pursue the death penalty against Bowers.

Bowers was a long-haul trucker who worked for himself, Brady said.

Little else was known about the suspect, who had no apparent criminal record but who is believed to have expressed virulently anti-Semitic views on social media. It appears Bowers acted alone, authorities said.

Stars of David with the names of those killed at the Tree of Life Synagogue. AP

The victims included Melvin Wax, a retired accountant in his late 80s who was always one of the first to arrive at synagogue and among the last to leave.

“He and I used to, at the end of services, try to tell a joke or two to each other,” said Myron Snider, a fellow member of New Light Congregation. “Most of the time they were clean jokes. Most of the time. I won’t say all the time. But most of the time.”

The toll also included professors, dentists and physicians.

Cecil Rosenthal, 59, and his younger brother David Rosenthal, 54, were intellectually disabled and lived together in Pittsburgh’s Squirrel Hill neighbourhood, near the synagogue where they were killed.

“Cecil's laugh was infectious. David was so kind and had such a gentle spirit. Together, they looked out for one another. They were inseparable,” said Chris Schopf, vice-president of residential supports for ACHIEVA, which helped the brothers live independently. “Most of all, they were kind, good people with a strong faith and respect for everyone around.”

Bowers apparently posted an anti-Semitic message on a social media account linked to him just a few minutes before he opened fire during Sabbath services on Saturday morning. After the attack, he told an officer, “I just want to kill Jews,” according to a federal affidavit.

In the basement, four members of New Light Congregation were just starting to pray — with two others in the kitchen — when they heard crashing sounds coming from upstairs, looked out the door and saw a body on the staircase, Werber, the survivor, recalled Sunday in an interview at his home.

He said Rabbi Jonathan Perlman closed the door and pushed them into a large supply closet. As gunshots echoed upstairs, Werber called 911 but was afraid to say anything, for fear of making any noise.

When the shots subsided, he said, Melvin Wax opened the door, only to be shot and fall back inside.

“There were three shots, and he falls back into the room where we were,” he said. “The gunman walks in.”

Apparently unable to see Werber and the other congregants in the darkness, Bowers walked back out.

A day later, Werber called the gunman “a maniac” and “a person who has no control of his baser instincts”.

“I don’t know why he thinks the Jews are responsible for all the ills in the world, but he’s not the first and he won’t be the last. Unfortunately, that’s our burden to bear,” Werber said. “It breaks my heart.”

People stand on the stairs of the Sixth Presbyterian Church as the crowd spills up the hill and down the street for a vigil. AP

The massacre came at a time of increasing anti-Semitism in the US. Earlier this year, the Anti-Defamation League reported a nearly 60 per cent rise in anti-Semitic incidents in 2017 — the largest single-year increase since the ADL began its tracking nearly three decades ago.

Perlman, who also spoke at the vigil Sunday night, said New Light lost three congregants.

“These three men, they cannot be replaced,” said Perlman, his voice breaking. “But we will not be broken. We will not be ruined.”