BERLIN — Investigators undertook a Europe-wide manhunt on Wednesday for a young Tunisian ex-convict with multiple aliases who had been denied asylum in Germany and was considered a security risk, linking him to a deadly truck rampage through a Berlin Christmas market.
The attack on Monday killed 12 people and wounded 48 — 12 of them seriously. The Islamic State has claimed responsibility for the assault, one of Germany’s deadliest acts of terrorism in decades. The aftermath has been complicated by a botched search for the driver, who has remained at large.
The revelations added to the growing pressure confronting Chancellor Angela Merkel of Germany, who decided last year to open the country’s border to roughly a million migrants and refugees.
A European arrest warrant identified the Tunisian as Anis Amri, 23, and said he had a history of providing false names and nationalities and should be considered armed and dangerous. German news agencies reported that the man had ties to Abu Walaa, a 32-year-old Iraqi Salafist arrested in Germany last month and accused of recruiting would-be jihadists to fight for the Islamic State. A reward of 100,000 euros, or about $104,000, was offered for information leading to his arrest.
It was not clear if the Tunisian was the actual driver. But the furious effort by the authorities to find someone who only months earlier faced deportation was outrageous, said Stephan Mayer, the home affairs spokesman for the conservative parliamentary bloc that includes Ms. Merkel’s Christian Democratic Union.
The German interior minister, Thomas de Maizière, confirmed that a manhunt was underway but would not get into specifics. “Success counts, and not speed or speculation,” he told reporters in Berlin.
Mr. Amri was recorded as having entered Italy in 2012, according to German news accounts. He traveled to Germany in July 2015 and applied for asylum in April this year, receiving papers that allowed him to stay in the country temporarily. He lived for a time in housing designated for asylum seekers in the city of Emmerich am Rhein, in North Rhine-Westphalia, Germany’s most populous state, and in Berlin.
At a news conference in Düsseldorf, Ralf Jäger, the interior minister of North Rhine-Westphalia, said that federal prosecutors had been observing the Tunisian man — he did not use Mr. Amri’s name — on suspicion that he might have been plotting an attack. When the man moved to Berlin in February, state authorities there picked up the monitoring.
The man was to have been deported in June, Mr. Jäger said. But because he did not have a valid passport, and because Tunisia did not initially acknowledge that he was a citizen, it was not possible to send him home. (Only on Wednesday did the Tunisian authorities issue a passport, he said.)
Mr. Mayer, the lawmaker, said the Tunisian man had spent a day in custody pending deportation, but because the authorities were unable to establish his identity “beyond doubt,” he was released. “This is a person who apparently was known to be potentially dangerous and who apparently was to be deported,” Mr. Mayer said.
In August, Mr. Amri was arrested in the southern city of Friedrichshafen with a fake Italian document and released a short while later, according to a law enforcement official who spoke on the condition of anonymity to discuss a continuing investigation.
An identity document found in a wallet left on the floor of the truck led the German authorities to seek the Tunisian man, said Frank Tempel of the Left Party. The document showed that the suspect had been allowed to remain in Germany but that he had not been granted full asylum.
Several of the men involved in the terrorist attacks in Paris in November 2015, in Brussels in March and in Nice in July were of Tunisian origin, but the number of Tunisians in Germany is low.
Mr. Amri is believed to come from the impoverished south of Tunisia, on the edge of the Sahara. His father told a Tunisian radio station, Mosaïque FM, that his son left Tunisia about seven years ago and served four years in prison in Italy after he was accused of setting fire to a school. The son was sentenced in absentia in Tunisia to five years in prison for violent robbery, the radio station reported.
Tunisians make up one of the largest groups of foreign fighters in Syria and Libya and have held leadership roles in the Islamic State. Tunisian security officials say the militants have been active in recruiting young volunteers in Tunisia and have links to immigrant networks in Europe.
A prominent Tunisian commander in the Islamic State, Boubaker al-Hakim, who was wanted in connection with terrorist attacks in Tunisia and was linked to the January 2015 attack on the satirical newspaper Charlie Hebdo in Paris, was reported killed last month in a drone strike in Raqqa, Syria.
According to the newspaper Süddeutsche Zeitung, the Tunisian man being sought in Germany had lived in the city of Dortmund with a man named Boban S., who has been arrested and accused of involvement with the Islamic State.
Boban S., in turn, is reported to have connections with Abu Walaa, the Iraqi who is known as “the man with no face,” because he often preached in Arabic and in poor German, with his back to the camera. The authorities arrested Abu Walaa on Nov. 8.
Abu Walaa, who has also been identified by officials as Ahmed Abdulaziz A., made his base in Hildesheim, a city of 100,000 south of Hanover, where he drew an increasingly devoted following and even offered his own app in 2014.
He was charged with recruiting terrorists and openly supporting the Islamic State, also known as ISIS or ISIL.
Officials emphasized that they were not yet certain that the Tunisian man had carried out the attack, and officials cautioned against jumping to conclusions. On Tuesday, the authorities arrested a 23-year-old Pakistani man who had applied for asylum in Germany, but they released him hours later, citing a lack of evidence.
Ms. Merkel and other officials have emphasized that they did not want the attack to jeopardize Germany’s commitment to a free and open society, and Foreign Minister Frank-Walter Steinmeier reaffirmed that idea on Wednesday. “We want to uphold this way of life, and not let it be destroyed by anyone, not even whoever was responsible for what happened here,” he told reporters.
Christmas markets in major German cities appeared to heed a call by Mr. de Maizière, the interior minister, to stay open. In Bonn, crowds continued to show up under the watch of the police. In cities like Dresden, the Berlin attack had its ominous presence felt with large obstacles to keep vehicles from getting among the stalls. The same was true in Vienna’s two famous markets, in neighboring Austria.
Laws and traditions in Germany strongly emphasize personal privacy, and the identities of the victims of the attack have barely begun to emerge.
One of the victims was a 65-year-old woman from Neuss, a city in North Rhine-Westphalia, according to news reports.
Fabrizia Di Lorenzo, an Italian transportation specialist who has been living in Germany for three years, has been missing since Monday, and her father, Gaetano Di Lorenzo, said he feared the worst.
“We are here with my wife, waiting for the DNA results,” he said in an interview with the Italian daily Corriere della Sera. “We are waiting for confirmation, but I am not deluding myself.”
The daughter’s cellphone and transit pass were found near the scene immediately after the attack, her relatives and friends reported on Tuesday on social media.
In Israel, Prime Minister Benjamin Netanyahu said that the attack “may have claimed the life of an Israeli citizen.” He was referring to Dalia Elkayam, who has been missing since Monday and whose husband, Rami Elkayam, was seriously injured in the attack.
Another victim of the attack was a Polish truck driver, Lukasz Urban, 37, who had a wife and young child and who was found dead in the cab of the truck. He had been stabbed and shot.