German police on Tuesday morning searched a large shelter for asylum-seekers in Berlin in response to a fatal attack on a Christmas market Monday. German media, citing unnamed security sources, reported that the suspect in the attack is a Pakistani man who entered Germany in late 2015 or early this year.

Police said Tuesday that the driver who rammed a truck into a crowded Christmas market in the heart of the German capital, killing at least 12 people and injuring nearly 50, did so intentionally and that they were investigating a “suspected terror attack.”

The dpa news agency reported that up to 250 officers took part in the raid at the shelter for asylum-seekers, at the now-defunct Tempelhof airport, overnight.

It quoted a spokesman for Berlin’s office for refugee affairs, Sascha Langenbach, saying four men in their late 20s were questioned by police but nobody was arrested.

Police declined to comment on the reports, referring questions to federal prosecutors who didn’t immediately respond to calls Tuesday.

The truck struck the popular Christmas market outside the Kaiser Wilhelm Memorial Church late Monday as tourists and locals were enjoying a traditional pre-Christmas evening out near Berlin’s Zoo station.

“Our investigators are working on the assumption that the truck was intentionally driven into the crowd at the Christmas market on Breitscheidplatz,” Berlin police said on Twitter.

“All police measures concerning the suspected terror attack at Breitscheidplatz are being taken with great speed and the necessary care,” they said.

Police stand beside a damaged truck which ran into crowded Christmas market in Berlin, Germany, Monday, Dec. 19, 2016. (AP Photo/Michael Sohn)

Police stand beside a damaged truck which ran into crowded Christmas market in Berlin, Germany, Monday, Dec. 19, 2016. (AP Photo/Michael Sohn)

Hours earlier Germany’s top security official had refrained from pointing to an intentional act, but said evidence pointed in that direction, while the White House condemned “what appears to have been a terrorist attack.”

The crash came less than a month after the US State Department called for caution in markets and other public places across Europe, saying extremist groups including Islamic State and al-Qaeda were focusing “on the upcoming holiday season and associated events.”

The Islamic State group and al-Qaeda have both called on followers to use trucks in particular to attack crowds. On July 14, a truck plowed into Bastille Day revelers in the southern French city of Nice, killing 86 people. Islamic State claimed responsibility for that attack, which was carried out by a Tunisian living in France.

After the Berlin attack, dozens of ambulances lined the streets waiting to evacuate people, and heavily armed police patrolled. Authorities on Twitter urged people to stay away from the area, saying they need to keep the streets clear for rescue vehicles.

Map locates Christmas market where the truck attack happened (AP)

Map locates Christmas market where the truck attack happened (AP)

Among the dead was a passenger in the truck, who succumbed as paramedics treated him, Berlin police spokesman Winfried Wenzel said. Police said later that the man was a Polish national, but didn’t give further details of who he was or what happened to him.

A suspect believed to be the driver was picked up about 2 kilometers (1½ miles) away, near the Victory Column monument. He was being interrogated, Wenzel said. The truck was registered in Poland, and police said it was believed to be stolen from a building site there. They didn’t give a specific location.

The Polish owner of the truck said he feared the vehicle, driven by his cousin, may have been hijacked. Ariel Zurawski said he last spoke with the driver around noon, and the driver told him he was in Berlin and scheduled to unload Tuesday morning. “They must have done something to my driver,” he told TVN24.

Federal prosecutors, who handle terrorism cases, took over the investigation, according to German Justice Minister Heiko Maas. In Washington, White House National Security Council spokesman Ned Price said the United States was in contact with German officials and ready to help in the investigation and response.

Firefighters look at a toppled Christmas tree after a truck ran into a crowded Christmas market and killed several people in Berlin, Germany, Monday, Dec. 19, 2016. (AP Photo/Michael Sohn)

Firefighters look at a toppled Christmas tree after a truck ran into a crowded Christmas market and killed several people in Berlin, Germany, Monday, Dec. 19, 2016. (AP Photo/Michael Sohn)

US President-elect Donald Trump blamed Islamist terrorists, though it was unclear what that assessment was based on. He said Islamic extremists must be “eradicated from the face of the earth” and pledged to carry out that mission with all “freedom-loving partners.”

Germany has not experienced any mass-casualty attacks by Islamic extremists, but has been increasingly wary since two attacks by asylum-seekers in the summer that were claimed by the Islamic State group. Five people were wounded in an ax rampage on a train near Wuerzburg and 15 in a bombing outside a bar in Ansbach, both in the southern state of Bavaria. Both attackers were killed.

Those attacks, and two others unrelated to Islamic extremism in the same week-long period, helped stoke tensions in Germany over the arrival last year of 890,000 migrants.

Firefighters look at a toppled Christmas tree after a truck ran into a crowded Christmas market and killed several people in Berlin, Germany, Monday, Dec. 19, 2016. (AP Photo/Michael Sohn)

Firefighters look at a toppled Christmas tree after a truck ran into a crowded Christmas market and killed several people in Berlin, Germany, Monday, Dec. 19, 2016. (AP Photo/Michael Sohn)