Later Merkel and German President Joachim Gauck attended a memorial service at the Kaiser Wilhelm Memorial Church and laid white roses outside the church.
Witnesses saw only one man flee from the truck after it rammed into the crowded Christmas market Monday evening. It smashed through the market, going 60 to 80 meters (200 to 260 feet) before finally coming to a halt.
Six of those killed have been identified as Germans, and the man found shot and killed in the truck's passenger seat was Polish. The other five people killed have not yet been identified, and 18 people are still suffering from serious injuries.
Merkel, who has been criticized for allowing in huge numbers of migrants last year, addressed head-on the possibility that an asylum-seeker was responsible for the carnage.
"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. "This would be particularly sickening for the many, many Germans who work to help refugees every day and for the many people who really need our help and are making an effort to integrate in our country."
A spokesman for Berlin's office for refugee affairs said police conducted a broad search overnight at a large shelter for asylum-seekers at the city's now-defunct Tempelhof airport. Four men in their late 20s were questioned but nobody was arrested, Sascha Langenbach told The Associated Press.
The Polish owner of the truck said he feared the vehicle may have been hijacked. Ariel Zurawski said he last spoke with the driver, his cousin, around noon on Monday and the driver told him he was in Berlin and scheduled to unload Tuesday morning.
"They must have done something to my driver," he told TVN24.
Germans have been increasingly wary since two attacks by asylum-seekers in the summer that were claimed by the Islamic State group. Five people were wounded in an ax rampage on a train near Wuerzburg and 15 wounded in a bombing outside a bar in Ansbach, both in the southern state of Bavaria. Both attackers were killed.
Those attacks, and two others unrelated to Islamic extremism in the same weeklong period, contributed to tensions in Germany over the arrival last year of 890,000 migrants.
Far-right groups and a nationalist party seized on Monday night's attack, blaming Merkel for what happened.
"Under the cloak of helping people Merkel has completely surrendered our domestic security," Frauke Petry, the co-chairwoman of the Alternative for Germany party, wrote.
Manfred Weber, a member of Merkel's conservative bloc and leader of the European Parliament's biggest political group, cautioned against sweeping verdicts but said it was important to ensure that extremists didn't enter the country.
The German government said Merkel spoke Tuesday with President Barack Obama, who expressed his condolences. In Washington, White House National Security Council spokesman Ned Price said the United States was ready to help in the investigation and response.
U.S. President-elect Donald Trump said Islamic extremists must be "eradicated from the face of the earth" and pledged to carry out that mission with all "freedom-loving partners."
The attack came less than a month after the U.S. State Department warned that extremist groups including Islamic State and al-Qaeda were focusing "on the upcoming holiday season and associated events" in Europe.
The Islamic State group and al-Qaeda have both called on followers to use trucks in particular to attack crowds. On July 14, a truck plowed into Bastille Day revelers in the southern French city of Nice, killing 86 people. IS claimed responsibility for that attack, which was carried out by a Tunisian living in France.
—Reuters contributed to this report.