German authorities have detained a Tunisian man they suspect of involvement in the truck attack on a Berlin Christmas market which killed 12 people, prosecutors have said.
The Islamic State militant group has claimed responsibility for the attack, which some critics say was made easier by Chancellor Angela Merkel's open-door migrant policy.
Investigators found the mobile phone number of the 40-year-old Tunisian stored in the phone of Anis Amri, the man believed to have hijacked a truck and rammed it into the crowd at a Christmas market in the German capital on Monday last week.
Amri, a failed asylum seeker also from Tunisia, was shot dead in Italy on Dec. 23.
Officials raided the home and business premises of the new suspect in Berlin. They did not name him.
"Further investigations indicated that he could have been involved in the attack," said prosecutors.
Prosecutors will decide by the end of Thursday whether to issue an arrest warrant for the man, now under temporary detention.
Investigators across Europe are trying to establish whether Amri had accomplices.
Germany's Focus magazine cited Berlin security sources as saying that on a phone found in the truck were voice messages and photos sent by Amri to Islamists 10 minutes before he steered the vehicle into the Berlin market.
Another priority is to piece together how he fled, undetected, from Berlin to Milan, taking advantage of Europe's open-border Schengen agreement. In Milan, police killed him after he pulled a gun on them during a routine check.
A spokesman for the Dutch National Prosecutor's Office said authorities were pursuing signs that the 24-year-old had passed through the Netherlands.
Dutch prosecutor Jirko Patist said investigators were studying images of a man they believe to be Amri taken by surveillance cameras at the train station in Nijmegen, near the Dutch border.
Dutch police were also investigating the possibility that Amri acquired a Dutch telephone SIM card he had with him at the time of his death.
"There is a possibility he got it at Nijmegen station," Patist said.
A French judicial source said on Tuesday Amri appeared to have passed through Lyon in France. Video surveillance footage showed Amri at the Lyon-Part-Dieu train station and officials believe he bought a ticket there for Chambery, between Lyon and the Italian border.
Political leaders including France's Marine Le Pen and Britain's Nigel Farage have said the case shows Europe's borders must not remain open.