The Tunisian authorities have arrested three men “linked to the perpetrator of the terrorist attack” at a Berlin Christmas market, including the suspect’s nephew, the government said in a statement on Saturday.

The three men, whose ages ranged from 18 to 27, were arrested on Friday. They were described as members of a “cell” that had been in contact with the suspect, Anis Amri, 24, a Tunisian citizen accused of killing 12 people on Monday when he plowed a stolen truck into a crowd of holiday shoppers.

Mr. Amri’s nephew, who was not identified by name, confessed to communicating with his uncle via the secure messaging app Telegram, swearing allegiance to the Islamic State and sending Mr. Amri money in the mail, according to the Interior Ministry statement.

Mr. Amri is the youngest of nine siblings, including five sisters. Two of his sisters, Hamida, 32, and Najwa, 28, said in an interview on Thursday that they did not believe that their brother was responsible for the attack. Another brother said he believed that Mr. Amri might have become radicalized while detained in an Italian prison.