The Berlin lorry attacker urged his teenage nephew to kill his uncle to prove his allegiance to Islamic State, according to Tunisian police.
Ferjani Fadi, 18, was arrested over the weekend in his hometown of Oueslatia, along with two others, believed to be cousins of the family, suspected of being members of a “terrorist cell” that supported Anis Amri.
Amri sent Fadi money to join him in Europe some time before he ploughed a truck into a Christmas market in Germany, killing 12, the Tunisian Interior Ministry said.
He also told Fadi to kill the husband of one of Amri’s sisters, a police officer, in order to prove his allegiance to Isil, a Tunisian police source told the Telegraph.
The method of pledging allegiance by committing murder echoes that used by Algerian terror groups in the 1990s.
Fadi is the son of one of Amri’s older sisters, and had been living in the capital Tunis where he was unemployed after dropping out of school.
The Tunisian interior ministry did not specify where the three suspects were arrested but said that the “terrorist cell” was “active” between Fouchana, south of Tunis, and Oueslatia.
All three men are being held pending investigation. An older brother of Amri’s was also taken in for questioning on Saturday morning and released.
On Saturday night police confiscated three computers from an internet cafe close to Amri’s family home which they believe may have been used by Fadi to communicate with his uncle in Germany.
Amri was killed in the early hours of Friday morning by police in Milan after a four-day manhunt across Europe.
His escape in the hours after the attack has raised questions about the German authorities’ response and Europe’s policy of open borders after it emerged that he had travelled through France and Germany.
Hundreds of German investigators are working through the Christmas period to establish the circumstances surrounding the attack, including whether Amri had help.
Spanish police are investigating whether Amri was in contact with a possible extremist there, on a tip from German authorities.
Amri’s nephew claimed his uncle was the leader of a jihadist group based in Germany known as the Abu al-Walaa brigade, linked to extremist preacher Abu Walaa, a known associate of Amri.
Amri fled Tunisia to Italy in the chaos after the so-called Jasmine Revolution in 2011 and spent four years in an Italian prison, where it is believed he may have been radicalised, before travelling to Germany.
On Sunday Tunisia’s security forces called on the government to take “exceptional measures” to combat the return of jihadists fighting for extremist groups abroad, after a protest in the capital in response to the Berlin attack.
Tunisia has been the largest source of foreign fighters joining Isil in Iraq and Syria, with between 5,000 and 7,000 having travelled to join the group.
About 200 people gathered in front of the Parliament in Tunis on Saturday holding banners that read “Close the doors to terrorism” and “No tolerance, no return.”
“The return of terrorists from hotbeds of unrest in Tunisia is worrying and could lead to the Somalisation of the country,” said a statement from the internal security forces’ national union.
Battle-hardened fighters “have received military training and have learnt to use all sorts of sophisticated weapons,” it added.