Authorities had ordered the Limbecker Platz mall in the western city of Essen shut on Saturday over "specific information regarding a possible attack".
Although there was no announcement of arms or explosives being found, Essen police had detained two men for questioning.
One man was subsequently released late Saturday, and the second on Sunday, "as there were no grounds for arrest," said the spokesman.
"However, this does not mean that the investigations are over," he added.
The shopping centre also reopened Monday.
Interior Minister Thomas de Maiziere on Sunday confirmed that there was a "link to the so-called Islamic State terror organisation".
"Some indications or orders" were given by someone who had travelled from Germany to a conflict zone in the Middle East, he added in an interview with public broadcaster ARD.
A German fighter belonging to Isis who is in Syria had sought to recruit several people for an attack in Essen, according to a source with knowledge of the case.
Investigators are examining if the jihadist is working with anyone in Germany, added the source, speaking to AFP on condition of anonymity.
Germany has been on high alert since last December's attack in Berlin, when a Tunisian national hijacked a truck and rammed it into a crowd, killing 12 people.
Domestic security officials estimate there are some 10,000 radical Islamists in Germany, with roughly 1,600 among them suspected of being prepared to commit violent acts.
Isis has claimed responsibility for several attacks in Germany in the past year, including the Berlin Christmas market truck assault, a suicide bombing in Ansbach, the murder of a teen in Hamburg and an axe rampage on a train in Wü0rzberg that injured five
Correction: A previous version of this incorrectly depicted a shopping centre in Oberhausen.