The gunman fired more than 100 rounds during the shooting.
Police also said that before the attacker went into the building, he fired 10 shots at a woman in a car parked outside – but she managed to get away and notify police.
Mr Grote called it a “horrific crime” and “very cruel”.
He said it was “something we’ve not experienced in the past – we see it on TV, we see it elsewhere, we’ve never seen it happen in our city”.
“It was the worst crime that our city has experienced recently.”
Police were alerted at 9.04pm local time on Thursday to the attack at the Jehovah’s Witnesses’ Kingdom Hall in the Gross Borstel district of Germany’s second largest city. Police received a total of 47 emergency calls about the incident.
A special operations unit of the police that was nearby arrived at the site at 9.09pm, with officials saying it was likely the unit’s fast arrival saved lives.
Hamburg police chief Ralf Martin Meyer said the man had a weapons licence and legally owned a semi-automatic pistol.
Officials also confirmed the gunman was unmarried and working in Hamburg.
Apart from the anonymous letter, police said no other reports were made against Philipp F previously and he had not committed any offences.
But officials said the gunman had reported a potential fraud to police, which is now being investigated by prosecutors.
Police also confirmed they searched the gunman’s apartment at 12.30am this morning, more than three hours after the shooting, and found another 15 loaded magazines and four packs of ammunition. Police also seized laptops and smartphones from the apartment.
In line with Germany’s privacy rules, police can not release the gunman’s full surname.
German Chancellor Olaf Scholz, a former Hamburg mayor, described the shooting as “a brutal act of violence” and said he was “speechless”.
Mr Scholz also said authorities “fear further victims may die from their severe wounds”.
