Police are searching for a new suspect in Monday's attack on a Berlin Christmas market, when a truck barreled into the crowded market and killed 12 people.

One man has already been detained and released. Police are now looking for a Tunisian asylum seeker with several aliases, North Rhine Westphalia Interior Minister Ralf Jaeger says.

The German government is offering a 100,000 Euro award (about $100,000) for information leading to the arrest of the man — who, among other names, is known as Anis Amri, NPR's Soraya Sarhaddi Nelson reports.

The 24-year-old is accused of carrying out the attack, and considered armed and dangerous, she says.

German media outlets, citing unnamed sources, report that Amri became a suspect after his temporary residency papers were found under the seat of the cab in the tractor-trailer.

"He was considered a threat and rejected for asylum by the refugee agency in Germany in June 2016 and was ordered deported," NPR's Soraya Sarhaddi-Nelson reports.

Specifically, Jaeger said, he was suspected of "preparing a serious act of violent subversion."

But Amri didn't have a valid passport, and Tunisia refused to acknowledge him as a citizen and give him fresh papers.

"So he stayed in Germany," Soraya reports. "That new passport from Tunisia arrived today, Jaeger says."

The person who was driving the truck appears to have been injured, and police have been searching hospitals in case the suspect sought medical treatment, German media report.

The search for the Tunisian suspect has been expanded across Europe, the BBC reports.

German police had previously arrested a Pakistani asylum-seeker, whom they picked up more than half a mile from the crash site. They later said that the eyewitnesses had been chasing the fleeing driver but lost sight of him before the Pakistani man was identified as a suspect. While the man they picked up matched the description of the truck driver, an investigation revealed no evidence tying him to the attack, prosecutors told the media.

That man was released on Tuesday.

The Islamic State has claimed responsibility for the attack, Soraya Sarhaddi-Nelson reports from Berlin.

"But in a late-night interview with public broadcaster ARD, Interior Minister Thomas de Maiziere says the government isn't buying the ISIS claim of responsibility, at least not yet," she reports.

"He said investigators are still following many leads — more than 80 at last count — and encouraged the public to give them a chance to do their job.

"But critics of Chancellor Angela Merkel are not waiting," Soraya says. "They blame her government's asylum policy for the attack and a few are demanding she step down."

The Associated Press reports that leaders in Berlin say security measures are being enhanced, but within limits:

"Berlin Mayor Michael Mueller said Wednesday it was 'good to see that Berliners aren't being intimidated.'" 'I don't think there's any need to be afraid,' he told ZDF television. 'The police presence has been significantly heightened ... and of course other measures taken to find the perpetrator quickly.'"Mueller argued that there are limits to increasing security, given the number of public spaces and events." 'It wouldn't be our free and open life any more if we escalated security measures so much that people worry about going anywhere, that there are strict entry checks,' he said. 'We don't want that. It must be appropriate and goal-oriented.' "Copyright 2016 NPR. To see more, visit http://www.npr.org/.