Police in the eastern German state of Brandenburg took a man back into custody Friday after releasing him on Thursday evening, a day after initially detaining him on suspicion of planning a terror attack in his home city of Eisenhüttenstadt, investigators said.

A raid on the man's home turned up no evidence of a terror plot, but an air rifle, military equipment and material supportive of the so-called "Islamic State" (IS) were found, police said.

The search of his apartment came after reports by residents that the man, a German national, had possibly been planning an explosives attack on a festival in Eisenhüttenstadt later this month. Police have said the suspect is known to them for drug crimes and is considered as "psychologically unstable."

'No concrete threat'

Prosecutors applied for an arrest warrant for him and another 30-year-old German national, who like the first is also a Muslim convert. Police said the two men, via an internet chat, had discussed strategies for carrying out attacks on people and objects, including possibilities for obtaining explosives.

They said the festival in Eisenhüttenstadt had been mentioned as a possible target, but that investigations so far indicated "no concrete threat to any persons, objects or events."

The arrest warrant was granted only for the 27-year-old, while the warrant against the 30-year-old was conditionally suspended.

German security services and police are on high alert after two IS-inspired attacks in the country in July.

tj/sms (AFP, dpa)