Islamic State extremists claimed responsibility Tuesday for the truck attack in Berlin that killed at least 12 people and wounded dozens more at a crowded Christmas market in the German capital.

The claim was posted late Tuesday on the website of the group's Amaq news agency. It said the attack was carried out "in response to a call for targeting citizens of the Crusader coalition."

The statement came a short while after German police released a lone suspect captured Monday evening after a huge truck smashed into an open-air market where locals and tourists were partaking in traditional Christmas festivities.

The Chief Federal Prosecutor's Office says it released the suspect, a Pakistani national, because investigators did not have enough evidence to charge him with a crime.

"The investigation up to now did not yield any urgent suspicion against the accused," the prosecutor's statement read.

German Chancellor Angela Merkel, who laid white roses at the attack site outside the landmark Kaiser Wilhelm Memorial Church on Tuesday, said she was "shocked, shaken and deeply saddened by the attack.

"Twelve people who were still among us yesterday, who were looking forward to Christmas, who had plans for the holidays, aren't among us anymore," she said in a nationally televised statement. "A gruesome and ultimately incomprehensible act has robbed them of their lives."

Earlier Tuesday, the head of Germany's Federal Criminal Police force said police have not yet found the gun believed to have been used to kill the truck's driver before it was stolen.