BERLIN — The Islamic State has claimed responsibility for the attack in which a truck plowed into a Christmas market outside Berlin’s Kaiser Wilhelm Memorial church, killing at least 12 people on Monday evening.

The announcement came after German authorities released a man whom they arrested on Monday evening, the Telegraph reported. The initial suspect is a 23-year-old Pakistani national, who was arrested at the Tiergarten park near the Christmas market shortly after the attack. He was released due to lack of evidence.

The passenger who was found dead in the cab of the truck has been identified as the original driver of the truck, 37-year-old Polish national Lukasz Urban. He appeared to have been stabbed and shot to death.

According to the Telegraph, Lukasz Urban was delivering steel from Turin, Italy to a warehouse in Berlin on Monday morning. However, the warehouse said they couldn’t unload the delivery and told Urban to return on Tuesday. Urban then waited in Berlin with the delivery. He talked to his wife at 3 p.m. before losing contact. It is believed that his truck was hijacked during this time.

The company that owns the truck said the truck’s GPS history shows unusual movements from 3:45 p.m. onwards, which indicate Urban was not in control. The truck seemed to have been driven by by someone learning how to drive.

German authorities are urging people to remain “particularly vigilant” and to report any suspicious movements, as the real culprit is still at large and possibly armed.