The 35-year-old, a German citizen and former Jehovah's Witness, used a semi-automatic pistol he had legally owned since December in the shooting on Thursday evening, a Hamburg state prosecutor said at a joint news conference with police.
The victims included four men and two women, and an unborn female child, the prosecutor said.
Hamburg police said the mother survived.
Officials said the motive remained unknown but a political reason had been ruled out.
The man had left the community voluntarily a year and a half ago, according to officials at a press conference on Friday.
Police who raided his flat after the shooting found 15 loaded magazines of ammunition.
The officials said about 50 people were at an event held in the Jehovah's Witness Kingdom Hall in the Alsterdorf district of the city when the shooting started.
When police arrived, the shooter ran to the floor above and shot himself, the officials said, praising police for a quick response they said possibly avoided more deaths.
The building, a nondescript block in a residential area, has been used by the group as a place of worship for several years, resident Annelore Peemueller told Reuters.
Phone footage from another resident showed a person outside the building shooting in through a window.
"I heard loud gunshots," said the witness, who declined to give his name.
"I saw a man shooting at a window with a firearm."
Television footage showed dozens of police cars as well as fire engines blocking off streets and some people, wrapped in blankets, being led by emergency service workers onto a bus.
"There were 12 continuous shots," another unidentified witness told reporters.
"Then we saw how people were taken away in black bags."
The mayor of Hamburg expressed shock.
"I extend my deepest sympathy to the families of the victims. The forces are working at full speed to pursue the perpetrators and clarify the background," Peter Tschentscher said on Twitter.
