BERLIN—German authorities on Thursday said they freed a man mistakenly arrested in connection with the Christmas market truck attack, marking the second high-profile release of a misidentified suspect and leaving open questions about whether the attacker had accomplices.

Germany’s federal prosecutor said the 40-year-old Tunisian arrested in Berlin the day before based on information found in the attacker’s cellphone had been falsely identified and was let go.

“The investigation into further accomplices, backers, confidants is pushing forward and we won’t slow down,” said Frauke Köhler, a spokeswoman for the federal prosecutor’s office.

Germany’s security forces have faced a storm of criticism over their handling of the suspect, Anis Amri, in the attack and its aftermath.

Nearly 24 hours after the Dec. 19 attack that killed 12, police released an initial suspect found to have no links to the crime, losing valuable time in their search for the attacker. Since then, security officials have been under increasing criticism that they knew Amri posed a threat but didn’t manage to stop him before the attack.

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