At least nine people were killed in Berlin Monday when a truck rammed into a crowded Christmas market.

The truck crashed into the German capital's popular Christmas market at the Kaiser Wilhelm Memorial Church. AP Television footage showed a large Scania truck with its windshield smashed out on the sidewalk alongside the market, and a swarm of ambulances nearby.

Police said they were still investigating whether the crash was an accident or an attack, but they said the driver was arrested nearby. The incident came less than a month after a U.S. State Department call for caution in markets and other public places that said extremist groups, including Islamic State and al-Qaida, were focusing "on the upcoming holiday season and associated events."

Islamic State and al-Qaida have both called on followers to use trucks, in particular, to attack public places.

Mike Fox, a tourist from Birmingham, England, told The Associated Press that the large truck just missed him as it drove into the market, tearing through tables and wooden stands.

"It was definitely deliberate," said Fox. He added that he helped people who appeared to have broken limbs. Others were trapped under Christmas stands, he said.

Heavily armed police patrolled the area after the crash. Police on Twitter urged people to stay away from the area, saying they need to keep the streets clear for the rescue vehicles.