A 43-year-old German man fatally shot the victims of immigrant backgrounds in the Frankfurt suburb of Hanau on Wednesday night before killing his mother and himself.
The man, named locally as Tobias Rathjen, left a number of rambling texts and videos espousing racist views and claiming to have been under surveillance since birth.
Meanwhile, officials confirmed they had received a letter from the suspect last November in which he sought help from authorities in stopping the surveillance he believed he was under.
The letter did not ring any alarm bells with prosecutors, authorities said.
Interior minister Horst Seehofer said state-level security officials and security agencies he consulted on Thursday agreed to increase the law enforcement presence around the country.
Mr Seehofer said there would be more surveillance at “sensitive sites”, including mosques, and a high police presence at railway stations, airports and borders.
“The threat posed by far-right extremism, anti-Semitism and racism is very high in Germany,” Mr Seehofer told reporters in Berlin.
Some also called for a crackdown on the extremist and anti-migrant ideology that has crept into mainstream political debate with the rise of the Alternative for Germany party (AfD).
A top official in the centre-left Social Democratic Party, a junior partner in Chancellor Angela Merkel’s governing coalition, accused AfD of providing ideological fodder to people such as the Hanau gunman.
“One person carried out the shooting in Hanau, that’s what it looks like, but there were many that supplied him with ammunition, and AfD definitely belongs to them,” Lars Klingbeil told German public broadcaster ARD.
Parts of Alternative for Germany were already under close scrutiny from Germany’s domestic intelligence agency.
The party has rejected all responsibility for far-right attacks, including an anti-Semitic attack on a synagogue and the killing of a regional politician last year.
One key question in the investigation is whether authorities or others were aware the suspect posed a threat.
“That’s one of the points that’s particularly interesting to me in this investigation,” said Mr Seehofer.
“Who knew what.”
Peter Frank, Germany’s chief federal prosecutor, said the investigation would concentrate on the suspect’s movements prior to the attack, and whether he had had contact with other people.
The suspect’s father was being questioned as a witness, he said.
Mr Frank acknowledged that his office had received a letter from the suspect three months ago.
The letter did not contain many of the more explicit racist and genocidal comments later found in the document posted on Rathjen’s website and did not prompt any action from prosecutors, Mr Frank said.
Mr Seehofer noted that rules intended to ensure stricter background checks on gun owners came into force on Thursday – a day after the attack – but that he was open to the idea of tightening the rules further.
One hurdle to effective communication between different branches of government in Germany is the country’s treasured notion of privacy, along with the complex web of local, regional, state and federal authorities that might be involved in checking a person’s suitability for gun ownership.
“We will always respect data protection,” said Mr Seehofer.
“But the discussion about data protection mustn’t push aside the discussion about security in this country.”