According to reports on Friday citing the arrest warrant, the 16-year-old Syrian refugee was instructed to place the bomb in a trash bin in a well-trafficked public place.
He afterwards was heard in a mosque saying that he wanted to kill for the Islamic State group.
Investigators are now in the process of evaluating and translating the chats, a spokesman for the state’s attorney’s office said.
This could take some time, he said, as it requires analysis of a “not inconsiderable amount of data” from the suspect’s mobile phone.
The 16-year-old asylum seeker was taken into custody Wednesday in the Porz district of Cologne, where he was living in refugee accommodation.
The suspect expressed “unequivocal willingness” to commit a bomb attack in online chats, Cologne police spokesman Klaus-Stephan Becker said during a press conference.