Three people including the nephew of the Berlin Christmas market attacker Anis Amri have been arrested in Tunisia.
Tunisia's Interior Ministry said Amri - who was shot dead on Friday - had sent his 18-year-old nephew Fedi money to join him in Europe.
Twelve people were killed and dozens more injured in the attack on Monday.
US President-elect Donald Trump branded Monday's deadly event in Berlin a "terror attack".
Taking to Twitter, Trump said: "Today there were terror attacks in Turkey, Switzerland and Germany - and it is only getting worse.
"The civilized world must change thinking!"
He also said, although not over Twitter, that "Islamist terrorists" slaughter Christians "as part of their global jihad".
German officials are yet to confirm whether they believe the crash was a deliberate attack or not.
The funeral for the lorry driver killed in the Berlin Christmas market attack has taken place in his home country of Poland.
Footage has emerged of the final moment before a truck ploughed into a crowd of people at a Christmas market in Berlin.
A man is being hunted by German police after a lorry ploughed into a Berlin Christmas market on Monday, killing 12, but who is the suspect?