The Berlin Christmas market suspect, Tunisian Anis Amri, had spent some time in Switzerland. Police are looking into whether the gun he used came from Switzerland.
The office of the Swiss Attorney General confirmed on Friday that Anis Amri had spent a period of time in Switzerland and may have made several trips to the Alpine country. The statement came after a report by Germany's ZDF public broadcaster.
The Swiss investigation "concerns information regarding logistics, people as well as the weapon used in the crime," the prosecution said in a written statement.
ZDF said investigators were particularly interested in whether Amri was in Switzerland when he obtained the gun used to kill a Polish truck driver ahead of the Berlin attack. They added that contacts found on Amri's mobile phone were being reviewed in Switzerland.
The 24-year-old drove a truck into revelers in the capital on December 19, leaving 12 dead. He also killed the Polish driver of the truck with an Erma pistol.
Amri was shot dead by Italian police on the outskirts of Milan four days later.
A police photo shows Anis Amri, the main suspect in Berlin's deadly Christmas market attack, at Milan's central train station on December 23
One line of investigation into the attack is whether Amri had any accomplices.
The Swiss inquiry involves suspicions that one or more people supported a criminal group and violated the ban on membership of terrorist militias, such as the so-called "Islamic State."
Police arrested another Tunisian man in Berlin this week, who prosecutors say had dinner with Amri at an Arab restaurant in the capital one day before the attack.
Amri had lived under at least 14 different names in Germany, a regional police chief said on Thursday.
Amri had been marked as a potential threat by authorities in the western state of North Rhine-Westphalia in February 2016, six months after he arrived in Germany and applied for asylum.
"He acted in a conspiratorial manner and used various personas," Dieter Schuermann, head of the state's Criminal Police Unit, told the regional parliament during a briefing.
jbh/sms (Reuters, dpa)