The three, aged between 18 and 27, were members of a “terrorist cell”, and they were detained overnight, the Tunisian interior ministry said.
Tunisian-born Anis Amri, 24, was shot dead by police near the Italian city of Milan on December 23.
This week’s truck attack at the Breitscheidplatz Christmas market left 12 people dead and 49 injured.
The interior ministry statement said Anis Amri’s nephew – the son of his sister – had confessed that he had communicated with his uncle via the encrypted chat application Telegram to evade security surveillance.
It said the three-member cell had been active in the towns of Fouchana, outside Tunis, and Oueslatia near Anis Amri’s hometown of Kairouan, about 95 miles south of the capital.
The statement added that Anis Amri had sent money to his nephew to travel to Germany and join a jihadist group, and encouraged him to pledge allegiance to ISIS.
Meanwhile, intelligence services in Spain are investigating a possible internet communication between Amri and a Spanish resident on 19 December, Interior Minister Juan Ignacio Zoido told radio station COPE.
On December 23, ISIS released a video showing Anis Amri pledging allegiance to its leader Abu-Bakr al-Baghdadi.
Anis Amri was shot dead after opening fire on police officers during a routine police check in the Milan suburb of Sesto San Giovanni, after a three-day Europe-wide manhunt.
According to a United Nations report, an estimated 5,500 Tunisians – mostly young people between the ages of 18 and 35 – were fighting in the ranks of terrorist organizations in Libya, Iraq, Syria and, to a lesser extent, Mali.
In November 2016, the ministry of the interior in Tunis said about 800 fighters had returned to the country.
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