BERLIN - The German capital was on high alert Tuesday with one or more attackers still at large in a deadly truck assault on a Christmas market, an act claimed by the Islamic State that struck at the heart of Europe's Christian traditions.

Chancellor Angela Merkel decried Monday's assault - in which a truck carrying a payload of steel plowed into festive stalls and fairgoers in Berlin, leaving 12 dead and dozens injured - as a presumed "terror attack," even as German police scrambled to find the culprit. The only suspect so far - a Pakistani asylum seeker taken into custody shortly after the bloodshed - was released by police Tuesday because of insufficient evidence.

Late Tuesday, the Islamic State, through the affiliated Amaq news agency, claimed that the attacker was a "soldier" responding to its call to target nations fighting the group in Iraq and Syria. The Islamic State has wielded the term before to describe lone wolves inspired by its rhetoric, and its level of involvement, if any, in coordinating the attack remained unclear.

In Germany and across Europe, revulsion and angst over the strike at a symbol of Christmas tradition sparked governments to act. The holiday spirit was being replaced by muscle.

Italy said it would ramp up security for Christmas events, including Pope Francis' appearance at St. Peter's Square in Vatican City. The Czechs pledged "massive" security at public events on Christmas and as the country rings in the new year. French officials said security at Christmas markets had been immediately reinforced, as its lawmakers observed a minute of silence for the all-too-familiar tragedy in Germany.

In Berlin, meanwhile, the release of the only suspect left police scrambling for fresh leads in the assault. They accelerated efforts to study forensic evidence, including analysis of bloodstains within the cabin of the truck - turned into a weapon in a tactic used five months earlier in a similar holiday rampage in Nice, on France's southern coast.

Investigation teams moved to piece together what they described as "circumstantial evidence," including witness descriptions and video footage. But no criminal sketches were released to the public, suggesting how much remained unknown. And as night settled over the German capital, Berliners were cautioned to stay on guard.

"It is the case that we possibly still have a dangerous offender in our area," warned Berlin's police chief, Klaus Kandt. "These days it is necessary to be vigilant."

The attack, officials concluded, was almost certainly deliberate.

A Polish national was driving the truck when it left Poland en route to Berlin to deliver a cargo of steel. The driver was found shot dead in the passenger seat.

The modus operandi and target, officials said, indicated - but offered no confirmation - that Islamist extremists were involved.

The Islamic State has previously cited traditional Christmas markets as viable targets in its wave of terrorism in Europe, and the Berlin assault was reminiscent of the truck-on-sidewalk tactic used by a self-proclaimed Islamic State adherent in Nice in July. That attack resulted in the deaths of 86 people on the Promenade des Anglais on Bastille Day.

Rita Katz, director of the SITE Intelligence Group, noted that the Islamic State's claim late Tuesday seemed unique. It is rare for the group to assert responsibility while the perpetrator is still alive. And unlike an official communique - such as those after attacks in Brussels and Paris - its claim through Amaq suggested, Katz said, "a little more distance." She said it could indicate that one or more attackers were inspired by the Islamic State but "had little to no coordination with the group."

German officials were assessing the group's claim. "This alleged confession of the so-called Islamic State - in fact they are a terror gang - we only just received," Interior Minister Thomas de Maizière told public broadcaster ARD. "We should let the security authorities do their work."

For Berliners, who live in a place often billed as Europe's most progressive city, the struggle Tuesday boiled down to perseverance - retaining a grip on classic German stoicism in the face of unfolding horror. In a country that eschews displays of patriotism, the colors of the German flag lit up the Brandenburg Gate. Flags flew at half-staff.

Late Tuesday, hundreds gathered near the attack site, around Kaiser Wilhelm Memorial Church - a symbol of the rebuilt, post-World War II Germany - to light candles, place flowers and listen to a service broadcast from within the house of worship.