Berlin attack suspect Anis Amri may have been killing quietly for MONTHS, German cops fear as they investigate ISIS murder mystery in Hamburg
The October attack was claimed by ISIS and saw a 16-year-old stabbed to death by lone-wolf knifeman
DESPERATE cops believe the Berlin Christmas Market massacre suspect could be linked to an unsolved ISIS murder.
Prime suspect Anis Amri was shot dead in a shoot-out with Milan cops in the early hours of this morning, reports emerging from Italy claim.
He is believed to have been behind Monday's atrocity that saw 12 people killed by a rampaging juggernaut.
And German police are now working on the theory an unsolved Hamburg murder that left cops baffled could have been carried out by Amri back in October.
"This is another lead we will follow," detectives confirmed.
The murder in Hamburg saw a 16-year-old named only as Viktor E knifed alongside the city's Alster lake.
His girlfriend was able to get away after being pushed into the water but Viktor died in hospital of his wounds.
Cops immediately began to work on the theory the killing had been an ISIS-inspired lone-wolf attack.
The group's AMAQ news agency wrote at the time: "A soldier of the Islamic State stabbed two individuals in Hamburg on the 16th of this month.
"He carried out the operation in response to calls to target the citizens of coalition countries."
An e-fit published shortly after appears to bear a resemblance to on-the-run Amri.
The revelation comes just hours after Berlin police announced they had CCTV footage of a man they believe to be Amri outside a Berlin mosque.
The footage was recorded just eight hours after Monday night's massacre.
The target was the 'Fussilet 33' association's building in Perleberger Straße in the south-east of the capital.
Neighbouring flats are also being searched, according to German media, with four people arrested this morning in Dortmund as police raided halfway houses and refugee shelters across the country to find associates of Amri.
The mosque was reportedly raided in 2015 over allegations it was raising money for extremists in Syria and an imam was put under investigation.
German federal police issued an arrest warrant for Amri today as it emerged the Tunisian was known to US authorities and was banned from flying to the country, reports the New York Times.
Police raided a refugee camp in Emmerich in the district of Kleve, Amri's most recent address, after a long delay due to problems with their warrant.
Authorities in Germany have come under fire after it emerged the suspect was under surveillance for months over fears he was planning an atrocity.
He also spent four years in jail in Italy after torching a migrant reception centre, according to his father.
And he was not revealed as a suspect until 24 hours after the attack on Monday night despite his ID being in the cab of the truck.
Police, who initially arrested and questioned the wrong man, reportedly did not find Amri's ID in the footwell of the lorry's cab until Tuesday - giving him a head start.
An £84,000 reward had been issued in the hope of tracking down Amri, Europe's most wanted man.
We pay for your stories! Do you have a story for The Sun Online news team? Email us at tips@the-sun.co.uk or call 0207 782 4368