The German authorities say they cannot be sure if a man in custody was behind Monday’s lorry attack on a Berlin Christmas market that killed 12 people.
“We have to entertain the theory that the detainee might possibly not have been the perpetrator,” federal prosecutor Peter Frank told reporters.
The style of attack and the target suggested Islamic extremism, he said.
The man detained, who has denied involvement, arrived in Germany from Pakistan at the end of last year.
He was captured in a park after reportedly fleeing the scene.
German Chancellor Angela Merkel, who laid flowers at the scene of the attack on Tuesday, vowed to punish those responsible “as harshly as the law requires”.
Her open-door policy on migration, which saw 890,000 asylum seekers arrive in Germany last year, has divided the country, with critics calling it a security threat.
Several attacks carried out by refugees in July had already heightened tensions.
At least 48 were injured, some seriously, in the attack, after the vehicle mounted the pavement at about 40mph and crashed into them.
A passenger in the lorry – believed to be the original driver – was later found dead inside. German authorities confirmed that the passenger was a Polish national and that he was not the person in control of the vehicle, which belonged to a Polish delivery company, at the time of the crash.
