BERLIN -- Police are searching for a Tunisian man in his early 20s in connection with the Berlin Christmas market attack, a German security official told CNN Wednesday.
The suspect's identity papers were found inside the cabin of the truck used in the attack, the official said.
Authorities released another man who was arrested shortly after the attack, which left 12 people dead and 48 injured, saying they did not have enough evidence to hold him.
Police have appealed to the public for any digital videos and photos they have of the incident or possible suspects. They tweeted Tuesday that they had so far received 508 leads.
Attacker on the loose
As the manhunt continues, ISIS claimed to have inspired the attack. The terror group's affiliated Amaq news agency described the perpetrator as a "soldier of the Islamic State," who had acted in response to calls for attacks in the West.
While German authorities are treating this as a terror attack, there is no evidence of any direct link between the attacker and ISIS.
According to German media, the man arrested and subsequently released was identified by witnesses as being the driver of the truck at the time of the incident, but police soon ascertained that this was not the case.
Peter Frank, general prosecutor at Germany's Federal Court of Justice, said in a statement that forensic tests did not link the man to the truck's cabin.
The US Embassy in Berlin said that terrorism "remains a concern across Europe, particularly during the holiday period."
"In light of the on-going investigation, Embassy Berlin encourages all US citizens to maintain good situational awareness, a low profile, and exercise vigilance," it said in a statement.
The UK Home Office warned its citizens to "remain vigilant and follow the advice of local authorities," in the wake of the attack.
Poland's ministry of the interior has asked military police to patrol the streets of Polish cities in the wake of the attack, according to local media. Police have no information indicating a terrorist threat in the country, reports said.
Original truck driver murdered
A Polish man found shot to death inside the truck has been identified as its original driver, according to German media.
He was not driving during the incident, police said. The gun used to kill him has not yet been recovered.
The truck, which was owned by a Polish company, was hauling 25 tons of steel before it was steered into the crowd, according to its owner Ariel Zurawski.
Zurawski identified the slain driver as his cousin. He said he was happy to be finishing his route from Italy to Berlin and was looking forward to returning to Poland soon.
"He was asking if he should be back home by Thursday night because he still needed to buy a Christmas gift for his wife," Zurawski said.
Country in mourning
German authorities have not publicly named any of the victims and police have asked people not to post videos or photos of them as a sign of respect.
But Italy's Foreign Ministry said Wednesday that Italian national Fabrizia di Lorenzo was missing following the attack and that her family was now in Berlin. The ministry said it was waiting for German investigators to complete their work before issuing a statement.
Italy's ANSA news agency reported that the missing woman, age 31, worked in Berlin and that her cellphone was found at the attack scene. Her mother and brother have gone to Germany to undergo DNA tests, ANSA said.
German President Joachim Gauck visited some of the injured at the Charité hospital in Berlin Wednesday morning.
He told reporters afterward that his visit was intended "to give support for the injured people, to show them the whole nation is supporting them" and that they are not alone as they fight for their lives.
German Chancellor Angela Merkel said she was "horrified and deeply sad about what happened" during a visit to the scene of the attack.
Dressed in black, Merkel called for unity in the face of the attack. She said it would be "especially disgusting" if the perpetrator of the attack turned out to be an asylum seeker, as police originally suggested.
Upheaval for Merkel?
Monday's attack could cause further political upheaval for Merkel, who has come under criticism over her government's generous acceptance of refugees. Germany has taken in more than 890,000 asylum-seekers in the past year, a much higher number than other European nations.
But a backlash has been growing, fueled in part by Islamist terror attacks in Germany and across the continent.
Christmas markets across the city were closed as Berliners gathered at the Kaiser Wilhelm Memorial Church near the scene of the attack to pay their respects to the victims.
Thousands of people both outside and in the church listened to the service, which included a song that asked: "So what are we fighting for? What do we live for? What do we pray for? What do we die for? What do we love for?"