The Christmas market is in a popular tourist neighborhood.
UPDATED: Dec. 20, 2016 10:30 a.m.
Berlin police told reporters Tuesday that they were uncertain the man they had in custody was the driver of the truck that rammed into a popular Christmas market Monday, killing 12 and wounding nearly 50.
"We have to entertain the theory that the detainee might possibly not have been the perpetrator," federal prosecutor Peter Frank said, BBC reported.
The suspect in custody is a Pakistani man who applied for asylum in Germany last year. He has continued to deny any involvement in what Chancellor Angela Merkel has called a "terrorist" attack.
Local authorities urged residents to be on guard as they continue to search for anyone involved in yesterday's incident.
A truck drove into a Christmas market in Berlin Monday evening, killing at least nine people and wounding dozens.
The market, located near Kaiser Wilhelm memorial church, is a popular holiday attraction.
Facebook activated its safety check feature to allow people to mark themselves safe after the suspected attack, which a member of parliament called “deliberate.”
Police said it appeared to have been a planned attack and not an accident, Reuters reported. A police spokesperson said that authorities had a suspect in custody who had attempted to flee the scene. At least one passenger was in the truck, and that person was pronounced dead on the scene, according to the Berlin police.
Chancellor Angela Merkel was “in mourning,” according to a spokesperson, and a member of Parliament in her Christian Democratic Union party condemned the incident as "deliberate."
“Although there is a host of unanswered questions, indications are it was a deliberate attack, carried out not just with the greatest brutality and disastrous consequences but also with a deliberate symbolism,” member of Parliament Wolfgang Bosbach said, in a statement.. “Just a few days before Christmas, in the middle of the German capital and amidst happy, peaceful people. The message is clear: no matter where, no matter how, we can pounce at any time.”
Germany has not previously seen a terror attack such as those that took place in neighboring France and Belgium in 2015 and 2016. Last year, terrorists attacked a concert hall and cafés in Paris, and a coordinated attack in Belgium killed 35 people. Most recently, a lorry attack in Nice, France, killed 86 people during celebrations for the French independence day July 14.