Authorities had ordered the Limbecker Platz mall in the central city of Essen shut on Saturday over "concrete information regarding a possible attack".
Although there was no announcement of arms or explosives being found, Essen police said two men had been detained for questioning in the nearby town of Oberhausen.
One man was later released, while the second was still being questioned, police said, adding that authorities were looking into "objects that were found in his possession".
Germany's domestic intelligence service had tipped police off about the threat, AFP learned Sunday.
A German fighter of the Islamic State group, who is in Syria, had sought to recruit several people for an attack in Essen, according to a source speaking on condition of anonymity.
Investigators are examining if the jihadist is working with anyone in Germany, added the source.
According to Bild daily, the IS fighter also provided instructions on bomb building in his online chat messages.
Separately, police in the southwestern city of Offenburg were deployed late on Saturday after a "possible threat of attack", citing a local disco as a potential target.
But they gave the all-clear on Sunday, without revealing further details about the threat. One man remains in custody over the case, said police.
Germany is 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 capable of violence.
The Islamic State has claimed responsibility for several attacks in Germany in the past year, including the Berlin Christmas market truck attack, the murder of a teen in Hamburg, a suicide bombing in Ansbach and an axe rampage on a train in Wuerzberg that injured five.