Media outlets identified the driver as a Syrian asylum seeker in his early 30s who arrived with the massive migrant influx to Germany in 2015.
The public prosecutor's office said it had opened an attempted homicide probe against the 32-year-old suspect, whose name was not released.
"The investigation into what was behind the act is ongoing," the prosecutors said in a statement.
"We cannot comment on the motive in particular -- we are pursuing all leads."
Public broadcaster ZDF quoted security authorities as saying they believed the incident had a "terrorist backdrop", but the prosecutor's office in Frankfurt declined to confirm this.
Germany has been on alert following several jihadist attacks in recent years claimed by the Islamic State group.
READ ALSO: Several injured after stolen truck slams into cars in Hesse
The most deadly was committed in 2016 by a 23-year-old Tunisian, who killed 12 people when he stole a truck and ploughed it through a Berlin Christmas
market.
The authorities remained cautious.
"There is still no confirmed information -- we are still in the critical phase of the investigation," a spokesman at the Frankfurt prosecutor told AFP.
He declined to confirm whether the suspect was Syrian.
A spokesman for the federal prosecutor's office in Karlsruhe, which usually takes over terror probes, said it was for now leaving the case to Frankfurt.
The white articulated lorry slammed into around nine cars at the red light opposite the Limburg courthouse in western Hesse state late Monday afternoon, crushing them together.
Police said "around nine people were slightly injured," including the
driver of the stolen vehicle, who was immediately taken into custody.
The red dot on the following map shows where the incident occurred in relation to the small city's main train station (Bahnhof) and cathedral (Dom).
'Didn't say a word'
News agency DPA citing security sources said the suspect was known to the police for drug offences and grievous bodily harm but had no apparent links to
the Islamist scene. Police searched his home in the nearby town of Langen overnight.
The original driver of the truck, who was not named, was quoted by daily
Frankfurter Neue Presse (FNP) as saying a man had "dragged" him out of his
lorry.
He said the man, with short dark hair and a full beard, had yanked open the
driver-side door and stared at him wide-eyed before forcing him out of the
vehicle.
"I asked him 'What do you want from me?'," he told FNP. "He didn't say a
word."
The truck sped into the parked cars a few hundred metres (yards) away and came to a stop on the central reservation of a six-lane road.
When the man behind the wheel of the truck emerged from the crash, several passers-by provided first aid, FNP said.
"The passers-by said the driver said 'Allah' several times" and spoke Arabic, FNP reported.
Police did not confirm this account.
Bettina Yeisley from Limburg, whose office is next to the scene of the crash, told FNP she heard a loud bang and ran out onto the street with colleagues.
They found the driver sitting by the roadside.
"He was bleeding from the nose, his hands were bloody, his trousers torn. He said that everything hurt. I asked him his name and he said, 'My name is Mohammed'."
Authorities urged users of social media not to jump to conclusions about
the motive.
"We are not ruling anything out," a spokesman for the state police force told DPA. "But we call on you: don't take part in speculation!" the regional police wrote in several tweets.
The Office for the Protection of the Constitution, Germany's domestic security watchdog, warned in April of an increased risk posed by IS fighters returning from Syria and northern Iraq.
Its director Thomas Haldenwang said an estimated 2,240 Islamists with "terrorist potential" were living in Germany.
The far-right Alternative for Germany party has seized on Chancellor Angela
Merkel's decision to leave the German border open to more than one million
migrants and refugees in 2015-16, accusing her government of compromising
national security.
It is now the biggest opposition party in parliament.
By Yann Schreiber and Deborah Cole
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