BERLIN - The day after a truck plowed into a bustling Christmas market in Berlin, killing 12 people, authorities are still scrambling for a motive, and the only person arrested after the attack was released because of insufficient evidence.

German Chancellor Angela Merkel described herself as “shocked, shaken and deeply saddened” after what she said the government must assume was a “terrorist attack.”

The Islamic State of Iraq and Syria (ISIS) has claimed responsibility for the attack. Terrorist monitoring groups report ISIS’ news agency made the claim.

Merkel, who has been criticized for allowing in large numbers of migrants, addressed head-on the possibility that an asylum-seeker was responsible.

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“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.

Authorities arrested a man about 1.5 miles from the crash site on suspicion of having been at the wheel of the truck. Footage showed the suspect, his head covered in a white sheet, being pushed into a police car shortly after the attack. Berlin’s Tagesspiegel newspaper reported that the man was known to police for minor crimes.

However, the suspect denies involvement in the attack, Berlin police said.

Below is the latest information on the attack, the investigation, and the world reaction to the worst terrorist attack in Germany since they began taking in migrants by the hundreds of thousands last year.

3:56 a.m. ET, Dec. 20 -- Police in Berlin said they had received 508 tips on the attack as of Tuesday night. But it wasn’t clear Wednesday whether prosecutors had any concrete leads.

8:44 p.m. ET -- Among the injured was Inaki Ellakuria, who underwent surgery Tuesday for a broken tibia and fibula on his left leg. He said he knew immediately it was no accident.

“It came fast, too fast to be driving off the road accidentally,” the 21-year-old student from Spain tweeted only minutes after the attack. “It has swept me and ran over both of my legs.”

Juan Jose Ellakuria told the Associated Press his son also suffered broken bones in his right ankle and instep, as well as damage to his hip.

“He’s come out of surgery and he’s making good progress,” Ellakuria said.

5:20 p.m. ET -- Brunella Capitani was walking with her husband to have dinner at the Berlin Christmas market when she heard the ominous sound of a truck plowing through stalls at the busy event.

Capitani told The Associated Press at her home in the northern Dutch city of Groningen that she heard the truck crash again and again “so there was this banging, repeating of the banging.”

She said she heard the truck but did not see it. “It was really like a part of a building that crashed down.”

The 48-year-old humanitarian volunteer said she was struck by the respect shown by those at the scene who did not immediately reach for their mobile phones to shoot photos or video of the carnage.

“It really touched me and I remember in particular the fact that we were not taking pictures and were not taking videos,” she said. “It’s like if we have already too much violence everywhere and you didn’t want to see more of this.”

5:18 p.m. ET -- A top Democrat on the U.S. House intelligence committee also says that ISIS’ claim of responsibility for a truck attack on a Berlin Christmas market cannot yet be confirmed.

Rep. Adam Schiff said the claim “cannot be verified until the perpetrator or perpetrators are apprehended by German authorities, but ISIS has never shied away from claiming credit for attacks - -even if it knew nothing about them in advance.”

Schiff warned that Europe must brace for the “possibility of further terrorism in the weeks and months ahead as Iraqi forces squeeze ISIS in Mosul and as the group is steadily dislodged from its strongholds in Syria.”

4:55 p.m. ET -- State Department spokesman John Kirby said U.S. officials don’t have enough information at the moment to back up the claim of responsibility by ISIS for a truck attack on a Berlin Christmas market that left 12 people dead and nearly 50 injured.

Kirby said “there is no direct evidence of a tie or a link to a terrorist organization, but this bears the hallmarks of previous terror attacks.”

The claim of responsibility came not long after German prosecutors said they’d released a Pakistani man picked up on Monday night near the scene of the attack, initially suspected of driving the truck.

4:48 p.m. ET -- London police will institute road closures in the area surrounding Buckingham Palace when the popular Changing the Guard military ceremony is taking place -- a security measure rushed into place because of the deadly Christmas market truck attack in Berlin.

Police said Tuesday the change in procedure had been planned but is being speeded up. The closures and the new physical barriers are set to be in place by Wednesday.

The Changing the Guard has long been popular with tourists and police say it is a high-profile event that attracts large crowds at an iconic location.

Under the new plan, road closures will be in place for nearly two hours when the guards are changed in front of the palace in central London. Police said the change is “a precautionary measure” for the next three months and is not based on any specific intelligence.

The threat level in Britain is judged to be “severe,” meaning an attack is highly likely.

2:44 p.m. ET -- Six of those killed in the attack have been identified as Germans, and a man found shot and killed in the truck’s passenger seat was Polish. The other five people killed have not yet been identified. Twenty-five people remained hospitalized, 14 with serious injuries.

2:26 p.m. ET -- The Islamic State of Iraq and Syria (ISIS) has claimed responsibility for the Berlin Christmas market attack. Terrorist monitoring groups report ISIS’ news agency made the claim.

Through it's 'Amaq News Agency, #ISIS claimed that the vehicular attacker in #Berlin #German is a "soldier of the Islamic State" pic.twitter.com/LZ7aJij3aG — SITE Intel Group (@siteintelgroup) December 20, 2016

ISIS has taken claim for numerous attacks in the past that they have not played a direct role in.

1:59 p.m. ET -- Federal prosecutors said witnesses were able to follow the truck’s driver from the scene but lost track of him. The man arrested -- a Pakistani citizen who came to Germany last year as an asylum-seeker -- matched witness descriptions of the truck driver, but investigators haven’t been able to prove that he was in the truck’s cab at the time of the attack.

They noted that witnesses were able to follow the truck’s driver from the scene but lost track of him. The man arrested matched witness descriptions of the truck driver, but investigators haven’t been able to prove that he was in the truck’s cab at the time of the attack.

Under German law, prosecutors have until the end of the calendar day following an arrest to seek a formal arrest warrant keeping a suspect in custody.

1:07 p.m. ET -- German prosecutors said the man who was arrested after Berlin truck attack was released because of insufficient evidence. He has denied any involvement, and a German newspaper reported earlier that no traces of the arrestees blood nor DNA were found in the truck used in the attack.

12:52 p.m ET -- Security is tightening at Christmas markets across Europe in response to the Berlin attack as pointed questions have arisen over why the German capital was vulnerable despite months of warnings.

Christmas markets are a popular tradition throughout Europe for both residents and tourists, and feature stalls with local food, mulled wine, warm chestnuts, scarves and tree ornaments.

With no concrete information available about the driver, security officials around Europe were left to draw their own conclusions about an attack with strong parallels to the Bastille Day rampage in France that killed 86 people and was claimed by ISIS.

French Christmas markets were already under heightened security. Paris’ Champs Elysees was lined with barriers and large concrete blocks straddled each entrance.

Concrete bollards - upright posts used to block entrances to streets - are increasingly common in France, which not only has struck by deadly extremist attacks in the past two years, but also endured a series of street bombings in the 1990s. Heavy barriers are relatively rare in Germany.

Germany’s interior minister left it up to local authorities whether to raise security after Monday night’s attack. In the eastern state of Thuringia and Potsdam, armed police patrolled around the markets and in the eastern city of Dresden, officers put up massive concrete blocks - and said they were planned ahead of the attack.

Italy and the Czech Republic promised heightened security to come, while Danish, Norwegian and British police increased their presence at Christmas markets.

11:35 a.m. ET -- Poland’s prime minister, Beata Szydlo, has called Lukasz Urban, a 37-year-old from the western village of Roznowo, “the first victim of this heinous act of violence.”

Urban’s colleague and cousin, Ariel Zurawski, the owner of the company that owned the truck that rammed into the market, said he was so dedicated to his work and his truck that he could be expected to defend the vehicle “to the end.”

Urban, who lived near the border with Germany, was found dead in the cabin of the truck at the market after the attack. It is believed the not-yet-identified driver hijacked the truck and shot Urban.

“It was really clear that he was fighting for his life. His face was swollen and bloodied. Police informed me that he had suffered gunshot wounds. Despite being stabbed he was shot dead,” Zurawski told Polish media.

Zurawski said Urban arrived with a delivery of steel at a branch of the Thyssenkrupp company in Berlin on Monday at 7 a.m. but was told to wait with his delivery until 8 a.m. the following day.

On Tuesday, Zurawski showed reporters a photo on his phone of his cousin in a kebab bar around 2 p.m., the last photo known of him still alive.

Berlin police chief Klaus Kandt said authorities have the “exact movement of the truck” from GPS but they are not giving details out and that it was only after the attack that the truck’s owner got in touch.

Zurawski said that Urban, who is survived by a wife and teenage son, last had contact with his wife at 3 p.m. local time, but that she couldn’t talk then because she was at work. She said she would call at 4 p.m., but at that point he was no longer answering his phone.

Zurawski described unusual movements on the truck’s GPS at 3:45 p.m. that indicate Urban was not in control.

10:05 a.m. ET -- Chancellor Angela Merkel and other senior German officials have laid white roses at the site of Monday’s attack on a Christmas market in Berlin.

Merkel was joined by Interior Minister Thomas de Maiziere, Foreign Minister Frank-Walter Steinmeier and Berlin Mayor Michael Mueller at the site near the Kaiser Wilhelm Memorial Church in central Berlin on Tuesday.

They spoke with police officers at the market then signed a book of condolences at the church.

9:46 a.m. ET -- Italy is strengthening security measures for areas where crowds are expected for Christmas ceremonies, such as Pope Francis’ appearance in St. Peter’s Square.

Interior Minister Marco Minniti led a meeting Tuesday of the heads of Italy’s police and intelligence forces to review security after the truck attack on a Christmas market in Berlin.

An interior ministry statement does not specify the places where security will be tightened. But security officials in the past have said that extremists have noted the Vatican, as a symbol of Christianity, is a logical target.

9:30 a.m. ET -- The head of Germany’s Federal Criminal Police Office said he cannot rule out that suspects involved in the truck attack on a Berlin Christmas market could still be at large.

Holger Muench told reporters Tuesday that authorities are still not positive the suspect they have in custody was the driver of the truck, that they have not yet found a pistol believed used to kill the truck’s passenger, and that it is not known overall how people were involved.

For those reasons, he said, we are “naturally on high alert and are investigating in all directions.”

He said six of the 12 people killed have been identified and are all Germans, but he does not yet know whether the other six are as well.

9:10 a.m. ET -- The Berliner-Zeitung newspaper is reporting that no DNA nor blood traces from the arrested suspect were found inside the truck that crashed into the market. Additionally, the newspaper reports there were no gunpowder traces found on the suspect, and police believe the truck driver shot the passenger in the truck, who later died. The information continues to point to the main attack suspect still likely being at large.

8:45 a.m. ET -- The owner of a Polish trucking company said the driver who was the first victim of the attack on a Christmas market in Berlin was stabbed and shot to death in the cabin of his truck.

Ariel Zurawski said German authorities asked him to identify the victim, Lukasz Urban, 37, from photos.

“His face was swollen and bloodied. It was really clear that he was fighting for his life,” Zurawski said, speaking to broadcaster TVN.

Lukasz Wasik, the manager of the trucking company, described Urban as a “good, quiet and honest person” devoted to his work.

“I believe he would not give up the vehicle and would defend it to the end if were attacked,” Wasik said in comments carried by TVP, Poland’s state broadcaster.

8:00 a.m. ET -- German newspaper Die Welt reports a top police official as saying “We have the wrong man.”

Law enforcement in Berlin have also publicly left open the door to the possibility that the truck driver may still be at large.

German officials have not said as much directly, but Berlin police continue to urge the public to be cautious and report tips. In a series of tweets they warn members of the public to continue to be vigilant as the investigation continues.

The temporary arrested suspect denies the offense.

Therefore we are particulary alert.

Please be also alert.#Breitscheidplatz — PolizeiBerlinEinsatz (@PolizeiBerlin_E) December 20, 2016

The BBC reports the head of Berlin police as saying they were “unable to confirm” whether the arrested suspect is the truck driver.