CEO Carsten Spohr said that the Airbus A320 in question has a code on the outside of the door that will open it "electrically and automatically" but it can be stopped by whoever remains in the cockpit.
"This can be impeded by those in the cockpit by pressing a lever that says lock and the door will be closed for five minutes," Spohr said.
The cockpit voice recorder was found in the wreckage and investigators have reportedly been able to hear the captain exiting the cockpit, leaving 27-year-old co-pilot Andreas Lubitz at the controls.
Tapping can be heard on the audio recording, progressing into louder banging, which is believed to be the captain trying to get back into the cockpit, Spohr said, noting that the company's planes had the doors to the cockpit reinforced so that access is not possible using force or weapons.
"Since the eleventh of September, the access to a cockpit has changed," Spohr said, referring to the 9/11 terror attacks in 2001.
Pilots and crew members know the code to get into the cockpit "by heart," Spohr said, suggesting that the co-pilot purposefully prevented the captain to get back inside.
Editor’s note: A French prosecutor initially said Lubitz was 28 years old but a law enforcement official later determined that he was 27.