BERLIN — German Chancellor Angela Merkel on Tuesday called the slamming of a truck into a Berlin Christmas market, killing 12 and wounding dozens, an assumed "act of terrorism," and acknowledged that the perpetrator may have been one of hundreds of thousands of migrants seeking sanctuary in Germany.

German authorities were questioning a male suspect arrested several hundred yards from the attack site. Two senior German officials briefed on the matter identified the man as a Pakistani national who arrived in February as an asylum seeker. Merkel — who came under fire by critics Tuesday for opening Germany's door to asylum seekers as well as to risk — conceded that the deadly attack may be tied to a would-be refugee.

Should it turn out that the attacker had been a refugee, Merkel said, this would be "particularly appalling to the many, many Germans who are actively helping refugees every day and to the many people who are indeed needing our protection and are making an effort to integrate in our country."

As security tightened across Europe following the attack, German police staged an unrelated counter terror operation Tuesday morning at a refugee shelter housed in Berlin's old Tempelhof airport, suggesting the possibility of further plots. On Tuesday morning, Merkel spoke to President Obama by phone and he pledged to U.S. aid in the German investigation.

The incident had echoes of the deadly truck assault in the French city of Nice in July that killed 86 people and was claimed by the Islamic State. Initially Germany officials had hesitated to use the word "attack." But early Tuesday police issued two tweets, one describing it as a suspected act of terror and the other declaring that "the truck was deliberately steered into the crowd" at the market."

If the suspect in custody is confirmed as the culprit, it is likely to add fuel to an already-heated debate in Germany over Merkel's decision to open the door last year to nearly 1 million migrants, most of them fleeing war in the Middle East.

She urged the nation not to give in to fear as Christmas approached with the threat of further incidents.

"We don't want to live with the fear of evil paralyzing us," said Merkel, who dressed in black as she spoke to reporters in Berlin. "We will find the strength for a life as we want to live it in Germany: free, united, and open."

But already, the debate in Germany over migrants was taking a sharper turn, and pundits were speculating whether the latest attack could damage Merkel's re-election bid next year.

Marcus Pretzell, chairman of the right-wing populist party Alternative for Germany (Afd), tweeted: "When will the German legal state strike back? When will this damned hypocrisy finally stop? These are Merkel's dead!"

Pretzell also shared a tweet by Justice Minister Heiko Maas announcing that flags at the ministry would fly at half mast alongside the comment: "If the government doesn't act soon, you can soon saw the masts in half."

The deadly incident occurred near the historic Kaiser Wilhelm Memorial Church in the Breitscheidplatz, a major public plaza. The bloodshed came at the height of activity at the Christmas markets, a cherished German tradition that draws locals and tourists to city squares for mugs of mulled wine, grilled sausages and regional sweets, as well as shopping at quaint stalls that sell handmade ornaments and other items.

In Berlin, the festive scene turned into panic shortly after 8 p.m. Monday as the truck veered onto the sidewalk and crashed between market stalls, running 50 to 80 feet before stopping, according to witnesses. Some victims were pinned under its wheels while others were struck and tossed onto the pavement.

According to police spokeswoman Valeska Jakubowski, 12 people were killed and 48 were being treated at hospitals.

After the incident, the scene remained a horrific tableau of crushed wood, broken glass and blood. Lying near the truck was a fallen Christmas tree, its star toppled.

On the scene, first responders were carrying people away on stretchers as police with automatic weapons cordoned off the area.

"It's terrible, just terrible," said Berlin Mayor Michael Müller. He said it was up to authorities to establish the facts, adding, "We always hoped that we wouldn't have this kind of situation in Berlin."

Following a rash of arrests, plots and attacks, Germany - like much of Europe - has been bracing for a terrorist assault on Christmas markets. Security had been beefed up, with guards checking bags at entrance points. So far this year, German officials have arrested more than a dozen people suspected of involvement in terrorist plots, while two others have carried out small-scale attacks.

According to Germany's DPA news service, the owner of the truck, Ariel Zurawski, told the Polish network TVN 24 that the vehicle was being driven by his cousin Monday and was carrying steel parts to Berlin. But he said he had lost touch with his cousin around 4 p.m. It was not immediately clear whether the truck had been hijacked. Officials said the man found dead in the truck was a Polish national.

Officials said they were exploring all possible explanations for the deadly incident, ranging from terrorism to a horrific accident.

"We cannot make any clear statement about the causes or the background" of the incident, said Winfried Wenzel, a spokesman for the Berlin police.

In a statement on Monday, U.S. National Security Council spokesman Ned Price condemned "in the strongest terms what appears to have been a terrorist attack on a Christmas Market in Berlin, which has killed and wounded dozens."

President-elect Donald Trump also denounced what he called a "horrifying terrorist attack in Berlin," saying in a statement that Islamist terrorists and their networks "must be eradicated from the face of the earth, a mission we will carry out with all freedom-loving partners."

Although Germany's Bild newspaper reported that authorities were investigating the attack as a possible act of terror, two senior security officials who spoke on the condition of anonymity said it was too early to determine the motive.

The killings in Berlin came just hours after a gunman in Zurich opened fire in an Islamic center, wounding three people who were praying, according to news reports. Police later found the gunman dead 300 yeards from the scene of the shooting.

In Berlin, a man who saw the bloodshed told the Berliner Morgenpost that the driver appeared to be targeting the market, turning off his lights as he steered toward the crowds. "It must have been on purpose because he didn't have the lights on," he said. "Then I just heard this loud bang and hysterical screams."

Breitscheidplatz is one of the busiest of the city's famous Christmas markets. Witnesses said visitors were drinking traditional hot wine and taking in the sights and sounds of the market when they heard a loud noise.

"We were enjoying the Christmas markets and some mulled wine," one witness, Emma Rushton, told CNN. "We heard a loud bang, and we started to see to our left Christmas lights were being torn down." At that point, she said, she saw a truck crashing through the crowd.

Following the incident, police said, witnesses reported that the driver fled the scene. The suspect, police said, was arrested near the towering Victory Column in Berlin's sprawling central park known as the Tiergarten.

The incident occurred as Germans have had to endure a growing threat of terrorism. So far this year, the Islamic State has claimed responsibility for two attacks in Germany.

In July, a 27-year-old rejected Syrian asylum seeker detonated a bomb near the entrance to a music festival in the center of the southern German town of Ansbach, killing himself and wounding several people.

Earlier that month, an Afghan asylum seeker armed with an ax injured four people on a German commuter train in the Bavarian city of Würzburg.

In July, a mass shooting in a Munich mall that wounded 36 and left 10 dead including the perpetrator was carried out by a mentally disturbed youth and was unrelated to Islamist terror.

Authorities for months have feared that terrorists might target Germany's famous Christmas markets.

German prosecutors last week said they were investigating an incident in which a 12-year-old boy allegedly plotted a nail-bomb attack at a Christmas market in the southern city of Ludwigshafen. According to German media, investigators said that they think the boy, who holds dual German and Iraqi citizenship, was guided by a member of the Islamic State.

Across Europe, officials were stepping up security. London's Metropolitan Police department, for instance, said Tuesday that it would review its plans for securing Christmas and New Year's celebrations following the Berlin attacks.

The plans, the department said in a statement, "already recognize that the threat level is at 'severe,' meaning an attack is highly likely, and have considered a range of threats, including the use of large vehicles."