By Cristina Silva, Mar. 26, 2015, IBTimes.com

Andreas Lubitz was not on any terror lists and had not been suspected of suspicious behavior. The 28-year-old German pilot allegedly brought down Germanwings Flight 9525 Tuesday, killing 150 people, after deliberately locking out his co-pilot from the cockpit, French Prosecutor Brice Robin of Marseille said Thursday.

Robin declined to describe the incident as terrorism or a suicide. Lubitz did not speak after the co-pilot left the cabin and was alive until the plane crashed, Robin said.

Carsten Spohr, CEO of Lufthansa, said there had been no reason to question Lubitz's performance. Lubitz was "100 percent fit to fly, without any restrictions," Spohr said. Lufthansa is the parent company of Germanwings.

Some expressed shock Thursday that prosecutors refused to call the plane crash an act of terrorism if they truly believed Lubitz was behind the mass killing. But terrorism experts say such labeling is often overused.