Walid Amri, the brother of 24-year-old Anis Amri, the Berlin Christmas market attacker, poses with a portrait of his brother in front of the family house in the town of Oueslatia in Tunisia | Fethi Belaid/AFP via Getty Images Berlin wants clarification on alleged police cover-up of terrorist’s past Christmas market attacker should have been arrested before December, says state interior minister.

The Berlin state parliament on Thursday demanded answers about an alleged police cover-up of the criminal past of Anis Amri, the Berlin Christmas market attacker.

The state's Interior Minister Andreas Geisel said Amri should have been arrested long before he carried out the attack in December and called for an investigation into the case.

It emerged on Wednesday that Amri was a known drug dealer and police allegedly had enough evidence to jail him up to six months before the attack in which 12 people died.

Prosecutors had believed that Amri was involved only in petty crime, based on police documents. But a newly released document suggests police had known about his drug trafficking past and after the attack they tried to alter the wording in their files to avoid blame.

A spokesperson for the Berlin police union told Die Welt that the claims harm the police's credibility.