"We haven't been able to confirm it yet," Berlin police chief Klaus Kandt told reporters Tuesday.

Thousands of locals and tourists were enjoying pre-Christmas celebrations in the popular tourist attraction Monday when the attack took place.

"It came in through the entrance, hit the sides of the barriers and then carried on past us," recalled Michael Fox, who witnessed the attack.

"It happened really, really fast. I thought it was just a runaway truck out of control, maybe somebody who was drunk, or didn't know how to drive," explained Shandana Durrani, an American witness.

Police say the driver of a delivery truck intentionally jumped the sidewalk at a high speed and ripped through shops and crowds.

German Chancellor Angela Merkel says authorities believe it was a terror attack.

"There is still a lot that we don't know about this act with sufficient certainty," she said. "But we must, as things stand, assume it was a terrorist attack."

Officials say there were two people in the truck. One was killed, and the driver fled the scene.

Although authorities have apprehended a suspect—a Pakistani citizen who came to Germany as a refugee in February 2016—it's unclear if he was the driver of the truck.

"I know that it would be particularly hard for us all to bear if it were confirmed that a person committed this act who asked for protection and asylum in Germany," Merkel said. "This would be particularly sickening for the many, many Germans who work to help refugees every day and for the many people who really need our help and are making an effort to integrate in our country."

In July, an ISIS terrorist killed 86 people when he rammed a truck into a crowd at a Bastille Day celebration in Nice, France. Since then, ISIS has encouraged followers to repeat truck attacks.

Last month, a man used a car to carry out an attack on the campus of Ohio State University.

In another brazen act of terrorism, a gunman in Turkey assassinated Russian Ambassador Andrey Karlov Monday in front of cameras while shouting "Allahu Akbar" and referring to the conflict in Aleppo, Syria. (Screengrab via CBN News)

Turkish and Russian leaders say the assassination was an attempt to disrupt efforts to repair ties between the two countries, which have been strained by the Syrian civil war.

The Daily Mail reports the shooter was also heard shouting: "We are the descendants of those who supported the Prophet Muhammad for jihad."

Turkish media say that phrase is similar to the anthem of Al Nusra, which is al Qaeda in Syria.

The gunman was later killed in a shootout with police.

Meanwhile, President-elect Donald Trump blames Islamic terrorism for the deadly violence in Turkey and Germany warning that, "The civilized world must change thinking."

Trump also said the Islamic State group "and other Islamist terrorists continually slaughter Christians in their communities and places of worship as part of their global jihad."

U.S., Homeland Security officials issued a warning that holiday celebrations could be targeted. In New York City, the NYPD is on high alert, posting armed guards at holiday markets.

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