A nephew of the Tunisian national suspected in the Berlin truck attack along with two others were arrested in Tunisia as part of a terror cell, officials said Saturday.

The unnamed nephew told police that his uncle — 24-year-old Anis Amri — made him swear allegiance to ISIS via the communication app Telegram, which they used to "escape security observation" because it is encoded, the Tunisian Ministry of Interior said.

Related: Berlin Truck Attack Suspect Anis Amri's Death Sparks Anger, Fear in Italy

Officials said the nephew confessed that Amri sent him money with another person's ID through the mail so that he could come to Germany to join him.

Amri was killed early Friday in a gun battle with police in Milan, Italy, after fleeing Berlin. He was being sought for Monday's rampage at a crowded Christmas market in which 12 people were killed and 48 others injured.

German officials have said Amri was in contact with an alleged ISIS recruiter and that he wanted his nephew to join the terrorist group. ISIS has claimed responsibility for the truck attack.

The nephew, who was arrested in Amri's hometown of Oueslatia, Tunisia, told police he adopted "fundamentalist ideology."

The two other unidentified suspects were arrested in Tunis. The three are between the ages of 18 to 27, and were all being held on suspicion of belonging to an organization linked to terrorist crimes. It's unclear whether any of them helped Amri escape police in Berlin.

Meanwhile, demonstrators took to the streets of Milan on Saturday, some calling for more security and scrutiny of foreigners.

Matteo Salvini, the leader of Italy's Northern League, stands behind a banner reading "No To Terrorism, More Security" in Milan on Saturday. Claudio Lavanga / NBC News

Italian police also provided some details of Amri’s journey to Italy, based on train tickets found on the slain suspect.

On Thursday evening, Amri took a train at at 5:24 p.m. (11:24 a.m. ET) from Chambery, France, to Turin, Italy, where he arrived at 8:18 p.m. (2:18 p.m. ET). It is not known how and when he got from Berlin to Chambery.

Amri then got on a train destined for Milan, Italian police said, arriving at 12:45 a.m. on Friday (6:45 p.m. ET Thursday). He walked to a bus stop nearby, and took a shuttle bus to Milan's Sesto San Giovanni, where he eventually encountered police.

While intelligence agencies are asking themselves how Amri managed to travel across Europe via train unnoticed, Italian authorities are looking to establish the reasons behind his decision to go to Sesto San Giovanni, a relatively quiet residential area.