Five years after a deadly lorry attack on Berlin's Christmas market in 2016, a memorial service was held in remembrance of the victims on Sunday.
President Frank-Walter Steinmeier said Germany must "clarify" the circumstances surrounding the Islamist terrorist attack.
"The state has not been able to keep its promise of protection, security, and freedom," Steinmeier added at the memorial anniversary in the Kaiser Wilhelm Memorial Church.
Germany "must correct these mistakes, where these happened, and it must further investigate when there are new findings," he went on.
Steinmeier's comments follow the release of an open letter from the families of the victims to new Chancellor Olaf Scholz, criticising shortcomings in the handling of the case.
In the letter, the families urged the government to order new inquiries into the individuals responsible for ordering and abetting the attack, an element they say has been under-explored.
Twelve people were killed when 24-year-old Tunisian Anis Amri drove a truck into the market on December 19, 2016, the deadliest Islamist attack to be perpetrated on German soil.
A 13th victim died this year having suffered serious injuries in the assault claimed by the Islamic State group.
An event "etched in our collective memory"
In a statement ahead of the memorial, Scholz said the events were "etched in our collective memory" and shared his condolences with the families of the victims.
Tributes to the victims took place at the Kaiser Wilhelm Memorial Church, whose bell tower, partially destroyed in World War II, overlooks the Breitscheidplatz square where the attack happened.
The church's bells rang out at 20:02 local time, the exact hour at which the truck drove into the Christmas market at full speed.
In his speech, Germany's president revealed there were "failures in the support offered to those left behind" in the weeks following the attack, another theme picked up on in the relatives' open letter.
The families call for the "dignified treatment for those affected" as well as the clarification of the circumstances of the attack.
"The Breitscheidplatz attack still raises a number of questions that have not been sufficiently answered in my opinion," Vice-Chancellor Robert Habeck said last Friday.
"The relatives of the victims have every right to be angry and ask for answers," Berlin's Interior Minister Andreas Geisel told local media.
On the run for four days after the attack, Anis Amri was eventually located and shot down by police in Italy.
Different inquiries highlighted errors in the surveillance of Amri, who arrived in Germany in 2015 and was quickly identified as a potentially dangerous Islamist and a drug trafficker.
Since 2000, law enforcement has foiled 23 such attacks, the Interior Ministry said in September.
German authorities remain on alert over the threat of further attacks, as the police revealed 554 individuals in Germany are considered to be dangerous Islamists.
IS also claimed responsibility for a knife attack in Hamburg in 2016, a bombing in Ansbach that injured 15 people, and an axe attack in Bavaria where five were hurt.
None of the assailants came to Europe carrying orders from IS, according to authorities. All of them seem to have organised their actions alone, sometimes under the influence of mental disorders.