A Tunisian man was detained over suspected links to the truck attack that killed 12 people at a Christmas market in Germany's Berlin last week, prosecutors said on Wednesday.
The 40-year-old Tunisian man's phone number was found stored in the phone of the believed attacker Anis Amri.
Officials searched his home and business premises but did not name the new suspect.
"Further investigations indicated that he could have been involved in the attack," said prosecutors, news agency Reuters reported. They will decide on Thursday whether to issue a formal arrest warrant for the man, now under temporary detention.
Amri hijacked the truck filled with steel rods and ploughed it into crowds at a Christmas market. A failed asylum seeker from Tunisia, Amri was shot on December 23 in Milan. The Islamic State claimed responsibility for the market assault.
A German magazine reports security sources as saying that a phone was found in the truck. Amri had sent photos and messages to Islamists 10 minutes before the attack through the same phone.
The Sueddeutsche daily said German authorities in Berlin had discussed Amri's case at least seven times between February and November. The newspaper also said it had seen documents showing Amri's search for instructions on building pipe bombs and explosive materials on the internet. The documents also showed that Amri had offered to be the Islamic State's suicide bomber.
Terrorism prevention experts discussed at least twice whether Amri was planning a specific attack in Germany, but both times decided it was unlikely, the paper reported, according to Reuters.
Investigators are trying to find out whether Amri had accomplices. There is also speculation as to how he fled from Berlin to Milan where he was shot.
The attack has intensified controversy about Merkel's migration policy less than a year before a federal election, and triggered calls for increased security measures, Reuters said.