Germany has acknowledged it was not prepared to deal with the aftermath of last year's attack on a Christmas market in Berlin that left 12 dead.
On the one-year anniversary of the attack, the justice minister has apologised for errors in the response.
Families have said they were not given timely information and that they were sent bills for the costs of autopsies.
Chancellor Angela Merkel will meet victims' relatives for the first time, after being accused of "inaction".
In an open letter before the face-to-face meeting was announced, several family members said the chancellor had failed to reach out to them.
"Almost a year after the attack, we note that you have not shared your condolences with us either in person or in writing."
Mrs Merkel has visited the scene of the attack several times, including once since the Breitscheidplatz Christmas market in central Berlin reopened earlier this month.
A report commissioned by the government and released last week cited a number of failures in the response to the attack, including delays in confirming the identities of the victims to their relatives.
On Tuesday, in an op-ed on Tagesspiegel newspaper (in German), Justice Minister Heiko Maas said the country was not "sufficiently prepared" for the consequences of such an attack.
"For this we can only apologise to the victims and surviving relatives. It remains a never-ending task of politics in a democratic constitutional state to learn from mistakes and to assume our responsibility towards the victims of terror."
He proposed the creation of a government co-ordination office to improve communication with victims of future attacks and called for a change in the law so that all victims could be treated and compensated equally, regardless of their nationalities or the circumstances of the attack.