Berlin truck killer Amri 'had 14 identities' in Germany
Anis Amri, who drove a lorry into a crowded Berlin Christmas market and murdered 12 people, was illegally registered under 14 aliases, according to German officials.
The revelation came from the head of criminal police in the state of North Rhine-Westphalia, where Amri had lived.
Dieter Schuermann said investigators did all they could to stop an attack.
But local authorities are accused of ending surveillance of Amri and allowing him to travel freely.
Opposition politicians have spoken of flagrant failures and misjudgements in handling him.
"The attack was committed by a man whom the security authorities knew a lot about," the state's interior minister Ralf Jaeger acknowledged at a hearing before the parliament in Duesseldorf.
Anis Amri, a 24-year-old Tunisian, had initially been registered in Kleve, North Rhine-Westphalia, but Mr Jaeger said he had moved to Berlin in February 2016.
'All legal powers exhausted'
His request for asylum was turned down in June, 11 months after he had arrived from Italy where he had served four years in jail.
But Tunisia refused to take him as he had no valid papers and he apparently took advantage of Germany's fragmented security apparatus to avoid scrutiny. Each of Germany's 16 states has its own police and security service, on top of the federal agencies.
The Duesseldorf hearing was told how state police had sought a prosecution against Amri in February for planning a serious act of violence against the state. The case was handed over to the Berlin authorities but there was no evidence of an imminent attack, the interior minister said.
Neither the federal nor the state security services had been able to provide sufficient evidence against Amri that would have stood up in court, said Mr Schuermann. Officials had "exhausted all legal powers to the limit to prevent potential dangers," he added.
Amri's use of multiple identities apparently enabled him to claim large amounts in welfare benefits. But it will also have hindered attempts to track him down. A suspect held on Wednesday on suspicion of helping Amri had also used false identities, officials said.
Amri was able to escape to the Netherlands on 21 December, apparently travelling from Berlin to North Rhine-Westphalia. He then took a train to Amsterdam before going on to Brussels, Lyon and Milan, where he was eventually shot dead.
Germany's federal interior minister has announced plans to tighten up its handling of failed asylum seekers and improve its monitoring of suspects identified as security threats. One measure likely to be accepted is a plan to detain individuals seen as potentially dangerous, known in German as "gefaehrders".