Anis Amri should have been arrested long before he hijacked a truck and drove it into a crowded Christmas market, killing 12 people, Berlin state Interior Minister Andreas Geisel announced on Wednesday.

Authorities had sufficient cause to arrest Amri on charges related to commercial drug trafficking, but failed to do so, Geisel told Berlin's senate. A newly revealed document first published in November showed Amri was involved in trafficking drugs, but was not arrested.

Geisel then alleged that staff at the state of Berlin's Criminal Police Office (LKA) tried to cover up their blunder after the deadly attack in December. Staff allegedly revised critical file notes and falsified documents to cover up their mistakes.

The interior minister said he has filed a criminal complaint and started an internal investigation.

Special investigator discovers incriminating notes

Geisel told the Berlin Senate that special investigator Bruno Jost had found two incriminating notes in the files of the LKA on Tuesday evening.

The notes revealed that Amri's phone had been monitored until June 2016, uncovering evidence of his involvement in the drug trade, and would have been sufficient to jail him. Authorities had judicial permission to bug his phone until October but stopped in June, and then posted the notes in November.

LKA officers wrote a file in January describing Amri's drug dealing as minor, but the file was backdated to November 1.

Amri's involvement with drugs had previously been reported to be relatively minor.

The December truck attack has caused widespread political fallout as politicians try to assign blame

Political fallout

Investigations into the handling of Amri's case ahead of the attack have caused widespread political fallout in Germany. In January, North Rhine-Westphalia's (NRW) interior minister, Ralf Jäger, claimed his state's intelligence agency had "gone to the limits of the law" in dealing with the Tunisian. But Federal Interior Minister Thomas de Maiziere told an NRW parliamentary inquiry in March that "one would have had good reason to apply for secure detention at the end of October at the latest. But it wasn't even attempted."

"Amri had entered the country illegally, had changed his place of residence without giving the foreigners' registration office an address where he was reachable, and there was a risk of flight at least because of multiple changes of identity," de Maiziere added.

A two-month Bundestag investigation found state and federal intelligence agencies failed to assess the potential danger that Amri posed, according to a secret report leaked to Berlin public broadcaster RBB,

The report found there were flaws in the way that Germany's joint terrorism defense center - an authority that coordinates state and federal terrorism intelligence - assessed potentially dangerous Islamists like Amri.