As speculation mounts that Monday's terrorist attack at a Christmas market in Berlin was carried out by a refugee, German Chancellor Angela Merkel has come under intense fire over her immigration policies.

Twelve people were killed and 48 injured when a 7-ton truck plowed into a crowded Christmas market on Monday evening.

While Merkel has previously admitted that terrorists used the flow of migrants to enter Europe, she has also strenuously defended her open-door immigration policy by stressing that Islamist extremism was already prevalent in Germany before the refugee influx.

Many Germans have taken to Twitter to lambast the chancellor for allegedly putting their country, and even the continent, in danger.

READ MORE: Refugees did not bring Islamist terrorism to Germany – Merkel

“The phenomenon of Islamist terrorism, of IS [Islamic State, formerly ISIS/ISIL], is not a phenomenon that came to us with the refugees, we rather have and had it,” said Merkel at an election campaign event in September.

Initial reports on Monday’s attack identified a 23-year-old refugee of Pakistani origin who arrived in Germany via the so-called ‘Balkan route’, a popular passageway for refugees, as the alleged attacker.

Many have marked the latest attack as the last straw for Merkel and called on her to resign. The chancellor is up for re-election between August and October 2017.

READ MORE: Berlin police detained ‘wrong man,’ truck attacker still at large, armed – reports

German police indicated on Tuesday that the detained individual has denied responsibility for the attack and warned that the person responsible may still be on the loose and possibly armed.