Police have arrested a man suspected of driving a truck into a crowd of people in Berlin Monday evening.

Police said it's unclear if the suspect was the driver of the truck, which plowed into a crowd of people outside a Christmas market in the German city, leaving 12 people dead and at least 48 others injured.

Another passenger in the truck died on the spot, police said.

Officials are investigating the crash as an act of terrorism, CNN reported Monday night.

The truck had Polish license plates, CNN reported, and the owner of the truck company told a private Polish television news network and CNN affiliate the truck may have been hijacked.

Fox News said the man came to Germany as a refugee from Pakistan this past February, according multiple German media outlets. Berlin police were encouraging citizens to stay at home and keep roadways clear in the aftermath of the attack. There was no evidence of other hazardous situations near the city. The German newspaper Der Tagesspiegel reported that the suspect was known to police for multiple minor offenses but was not being tracked by anti-terror authorities.

Im Nahbereich #Breitscheidplatz wurde eine verdächtige Person festgenommen. Ob es sich um den Fahrer des LKW handelt, wird derzeit geprüft. — PolizeiBerlinEinsatz (@PolizeiBerlin_E) December 19, 2016

Der Beifahrer des LKW, der am #Breitscheidplatz in den Weihnachtsmarkt gefahren ist, verstarb vor Ort. — PolizeiBerlinEinsatz (@PolizeiBerlin_E) December 19, 2016

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The White House said the incident "appears to have been a terrorist attack" and offered assistance to German investigators.

"Germany is one of our closest partners and strongest allies, and we stand together with Berlin in the fight against all those who target our way of life and threaten our societies," National Security Council spokesman Ned Price said in a statement.

President-elect Donald Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE released a statement Monday night condemning the attack by "Islamist terrorists."

"Innocent civilians were murdered in the streets as they prepared to celebrate the Christmas holiday," the statement read. "ISIS and other Islamist terrorists continually slaughter Christians in their communities and places of worship as part of their global jihad. These terrorists and their regional and worldwide networks must be eradicated from the face of the earth, a mission we will carry out with all freedom-loving partners."

In July, a man drove a truck along a beach front in France as people watched fireworks on Bastille Day. Eighty six people died in that attack, which the Islamic State in Iraq and Syria (ISIS) took credit for.

Updated 9:10 p.m.