Authorities said the attacker had stored several gas bottles and canisters with gasoline and bio-ethanol at his home.
He was shot dead by Italian police in Milan four days later while on the run.
"With dismay, I learned of the bad incident in Muenster", Seehofer said in a statement Saturday, adding, "Police in Muenster and throughout (the state of North Rhine-Westphalia) are now working hard to investigate the matter".
Berlin police tweeted that six people were detained in cooperation with the city's prosecutor's office. At this time, authorities believe that the main suspect was motivated to take revenge for Amri's death, which occurred after he fled the scene of his attack on foot and was shot by police.
The Sueddeutsche Zeitung said the man had psychological problems. Six of the 20 injured were in critical condition.
The prosecutor's statement said no one was detained during the raids. It didn't say why.
"We have no indications that there is a political background or that others were involved" in Saturday's deadly crash, prosecutor Elke Adomeit told reporters. His run-ins with the law regarded threats, property damage, fraud, a hit-and-run and domestic conflicts with his family, but Adomeit said that all charges were dismissed.
Memorial grows in Crown Heights for man fatally shot by police
When the officers arrived, police said , Vassell took a two-handed shooting stance and pointed an object at them. Footage shows a man that police believe to be Saheed Vassell holding a gray object and pointing it at people.
German officials lay flowers at a fountain in Muenster after the attack..
Armin Laschet, the governor of North Rhine-Westphalia state where Muenster is located, toured the city Sunday. "And also the people of The Netherlands, who were sitting here and became victims".
The German news agency dpa reports that a large-scale police operation is underway and that much of the area is cordoned off to the public. Twelve people, including the man driving the truck when it was hijacked, were killed.
A body bag on a stretcher is being loaded into a van as investigators work at the square where a man rammed his vehicle into a crowd killing two and injuring several others the night before in Muenster, western Germany on April 7, 2018.
Police investigators were still trying to learn the man's motive.
Local media have described the 48-year-old as a psychologically instable man.
German forensic police combed the area outside a restaurant in the western city of Muenster on Sunday as they tried to establish the motive of a man who drove a camper van into a group of diners, killing two before shooting himself dead.