German authorities received warnings last year about the suspect in a car attack at a Christmas market which killed five people, officials have said.
Authorities have identified the suspect as a Saudi doctor who arrived in Germany in 2006 and had received permanent residency.
Police have not publicly named him, in line with privacy rules, but some German news outlets have identified him as Taleb A, and reported that he was a specialist in psychiatry and psychotherapy.
The head of the Federal Criminal Police Office, Holger Munch, said in an interview on the German broadcaster ZDF that his office received a tip-off from Saudi Arabia in November 2023, which led authorities to launch “appropriate investigative measures”.
He added: “The man also published a huge number of posts on the internet. He also had contact with various authorities, made insults and even threats. However, he was not known to have committed acts of violence.”
He said the warnings were very unspecific.
Meanwhile, the German Federal Office for Migration and Refugees said on social media site X it received a tip in late summer 2023.
It said: “This was taken seriously, like every other of the numerous tips.”
The office highlighted it is not an investigative office and said it referred the information to the relevant authorities, following procedure in such cases.
It gave no other details about the suspect or the nature of the warnings.
Interior minister Nancy Faeser said on Sunday: “This perpetrator acted in an unbelievably cruel and brutal manner – like an Islamist terrorist, although he was obviously ideologically an Islamophobe.
“The views and statements made by the perpetrator are being investigated, as are the tips and procedures that were given to various authorities and the judiciary. The right conclusions must then be drawn from this.”
The update came as police in Magdeburg, the central city where the attack took place on Friday evening, confirmed on Sunday that those who died were four women aged 45, 52, 67 and 75, as well as a nine-year-old boy they had spoken of a day earlier.
Authorities said 200 people were injured, including 41 who are in a serious condition. They are being treated in multiple hospitals in Magdeburg, which is about 80 miles west of Berlin, and beyond.
The suspect was on Saturday evening brought before a judge, who behind closed doors ordered that he be kept in custody pending a possible indictment.
Authorities say he does not fit the usual profile of perpetrators of extremist attacks.
Describing himself as a former Muslim, the suspect appears to have been an active user of the social media platform X, sharing dozens of tweets and retweets daily focusing on anti-Islam themes, criticising the religion and congratulating Muslims who had left the faith.
He also accused German authorities of failing to do enough to combat what he referred to as the “Islamification of Europe”. He appears to have been a supporter of the anti-immigrant Alternative for Germany party.
Yet another act of mass violence in Germany makes it likely that migration will remain a key issue as German heads towards an early election on February 23.
Alternative for Germany has been polling strongly amid a societal backlash against the large numbers of refugees and migrants who have arrived in Germany over the past decade.
Right-wing figures from across Europe have criticised German authorities for having allowed high levels of migration in the past, and for what they see as security failures now.
Hungarian Prime Minister Viktor Orban, who is known for a strong anti-migration position going back years, used the attack in Germany to lash out at the European Union’s migration policies.
At an annual press conference in Budapest on Saturday, Mr Orban insisted: “There is no doubt that there is a link between the changed world in Western Europe, the migration that flows there, especially illegal migration and terrorist acts.”
He vowed to “fight back” against the EU migration policies “because Brussels wants Magdeburg to happen to Hungary, too”.
