Hans-Georg Maassen told public broadcaster ARD that intelligence leads had suggested in early September that militant group Islamic State (IS) was planning an attack on Germany's transport infrastructure.
Spies managed to track down and identify the suspect in the eastern state of Saxony on October 6 and started a round-the-clock observation, Maassen said.
"We found out that he then bought hot glue in a discount shop on the following day. And then we immediately put all measures into place to start a raid because we assumed this can basically be the last missing chemical for him to build a bomb."
Maassen told ARD that initial intelligence information pointed to an attack on trains in Germany but it later became clear that he planned to strike a Berlin airport.