Axel Schmidt/Reuters
A German policeman holds the picture of a terrorist-subject infront of the main terminal of Berlin-Schoenefeld airport, in Schoenefeld, near Berlin, October 9, 2016, following a suspicion that a bomb attack was being planned in Germany
Spiegal reports that the man, Jaber Albakr, had been under round-the-clock surveillance due to a high risk of suicide and hunger strike, so it was not immediately clear how he was able to kill himself.
Spiegel reports that Albakr hanged himself in his jail cell.
German police notified the public that they were looking for Albakr - a Syrian who had been granted entry into Germany in 2015 - on Saturday for amassing explosives in his apartment in the eastern town of Chemnitz.
Two other Syrian refugees who recognized him from photos on social media captured him and tied him up at their home until police arrived on the scene.
"We succeeded in preventing a terrorist attack just minutes before midnight," Hans-Georg Maassen, head of the domestic intelligence agency, told German broadcaster ZDF Police on Tuesday. Albakr had evidently been evading police capture since Saturday.
Germany's spy chief said on Tuesday that Albakr had ties to ISIS, but that has not yet been confirmed.