BERLIN — A Pakistani migrant who was arrested Tuesday as a suspect in the deadly truck attack on a crowded Christmas market in Germany has been released because of insufficient evidence — meaning authorities are still on the hunt for the perpetrator.

The man was arrested not far from the scene of Monday night's carnage in the German capital, where 12 people were killed and nearly 50 others wounded.

But the Federal Prosecutor's Office released him Tuesday night after investigators said they could not prove he was in the cabin of the truck during the rampage.

Police in Berlin tweeted that they received more than 500 leads from the public and that they working intensively with Federal Police on this case.

Police were stepping up security at Christmas markets across the city, and similar measures were being taken across the country. Investigators planned to return to the scene on the attack on Wednesday.

The ISIS-affiliated news agency Amaq claimed responsibility for the attack, and said a soldier of ISIS carried it out as part of the terror group's call to target people internationally. The news agency gave no proof that ISIS was actually involved and German officials did not immediately respond to the statement.

U.S. State Department Spokesman John Kirby said Tuesday afternoon there was “no direct evidence of a tie or a link to a terrorist organization” but the attack "bears the hallmarks" of previous terror attacks.

The suspect entered the country last year and had applied for asylum, Interior Minister Thomas de Maiziere said, adding that other people may be involved.

The suspect in custody has denied responsibility, and police were not sure if he was the main culprit.

Related: Boy, 12, Tried to Bomb Christmas Market, Officials Say

Berlin police chief Klaus Kandt earlier told reporters: "We are not sure if the suspect in custody is the right man ... in fact, in my view it is still not certain whether he really was the driver."

Germany's chief prosecutor Peter Frank echoed that, warning: “We must get used to the idea that he was possibly not the perpetrator or that he didn't belong to the group of perpetrators.”

He added: "We do not know if there were more people involved in a support role."

A temporary accommodation center for migrants was raided overnight by special operations police, but no further arrests were made.

Chancellor Angela Merkel, who has faced political pressure over open-arms policy for refugees, earlier said it would be "hard for us all to bear” if the perpetrator "was someone who sought protection and asylum."

She added: “I have no simple answers. We cannot live without these shared moments in public spaces. We must not be limited by fear of evil. We will live as we wish in Germany — free, together and openly.”

More than 2.1 million immigrants were registered in Germany in 2015, according to provisional results from the Federal Statistics Office. That represented a 46 percent year-on-year increase and is almost as many people who live in Houston, the fourth-largest city in the U.S.

Federal prosecutors took over investigation after the truck was driven into Breitscheidplatz, a popular public square where tourists and locals were enjoying a traditional pre-Christmas evening.

Police said the truck was "intentionally directed" into the crowd in an act of “probable terrorism.”

Merkel visited the scene, adding to a growing pile of flowers left in tribute to the victims.

Berlin Mayor Michael Mueller, German Chancellor Angela Merkel and German Interior Minister Thomas de Maiziere visit the scene of the market attack in Berlin. HANNIBAL HANSCHKE / Reuters

Local media identified the suspect as “Naved B.,” a 23-year-old who entered Germany via Austria on Dec. 31, 2015. He was reportedly already known to police for minor offenses. Those reports could not immediately be confirmed by NBC News.

De Maiziere said only a few of the victims had been identified so far, and that 18 of the 48 wounded had suffered severe injuries.

The U.S. State Department said on Tuesday afternoon that they were not aware of any American citizens killed or injured in the attack.

Among the dead was a Polish man found shot to death inside the cab of the stolen truck. The weapon has not been found.

Bloodstained clothing was also found inside the cab, but the suspect in custody was wearing clean clothes, Frank said.

Christmas markets in Berlin were closed Tuesday as a mark of respect for the victims, but the interior ministry said other events around the country would take place with increased security measures.

Emma Rushton, an English tourist who witnessed the incident, told NBC News that she heard the "terrifying" crash while she was with a friend at the Christmas market.

"People were bleeding. There was lots of blood and lots of crying," she said.

Rushton said she that thought the truck was going about 40 mph and that "there was no way it was just a veer-off-the-road accident."

Other witnesses said victims were crushed.

The owner of the truck said he feared the vehicle may have been hijacked. Ariel Zurawski said he last spoke with the driver around noon on Monday, and was told the vehicle was in Berlin and scheduled to unload Tuesday morning. "They must have done something to my driver," Zurawski told TVN24.

The crash came less than a month after the State Department warned U.S. citizens to exercise caution at overseas holiday festivals and outdoor markets in light of the heightened risk of terrorist attacks throughout Europe.

In July, a truck driver killed 84 Bastille Day revelers in Nice, France, and injured more than 200 others before he was shot dead by police. Officials said he had been planning the attack for months.

Related: Low-Tech, Soft Target Terrorism Is Growing Threat

German officials said Merkel spoke with President Barack Obama by phone Tuesday about the Berlin attack.

"President Obama assured the German chancellor that the United States would support Germany in the investigation," Merkel's spokesman Steffen Seibert said in a statement.