The investigation file itself shows that this was not the case
Various press reports have claimed that there was an intentional lockout. This has led to the assumption that immediately after the captain left the cockpit Andreas locked the door. Over and over again pictures of the cockpit door locking system switch were commented on together with pictures of the UNLOCK-NORM-LOCK cockpit panel. Media websites displayed A320 locking system animations, always with the claim that Andreas had set the switch to «LOCK».
According to the BEA final report (see page 12) on the accident of the Germanwings plane, the cockpit door was opened at 09:30:24 and closed again three seconds later. These three seconds are when the captain supposedly exited the cockpit. This agrees with the transcript of the cockpit voice recorder protocol. See this following excerpt from the investigation file:
Objective findings German translation volume_03_page HA 05221:
Approximately four minutes later, at 09:34:31, a ringtone was recorded for one second. This sound indicates that someone outside the cockpit is requesting access. This is also evident from the BEA final report on page 13 and is consistent with the cockpit voice recorder transcript recordings (see excerpt below from the investigation file).
Objective findings German translation volume_03_page HA 05222:
However, the recording of the ringtone access request proves that Andreas had not deliberately locked the cockpit door. This is because if he had deliberately set the toggle switch to the LOCK position the entire cockpit door electronics, including the keypad and audible ringtones, would automatically have been deactivated for a period of 5 to 20 minutes, depending on the airline. Thus, no ringtone would have sounded and no ringtone could have been recorded. However, the fact that the ringtone was recorded is clear from the BEA final report. This clearly refutes the claim that by deliberately locking the cockpit door Andreas prevented the captain from entering!
Finally, reference should be made to finding no. 5 in the report on the crash of Germanwings Flight 4U9525 by Mr van Beveren:
The assumption stated in the report and also by the public prosecutor, Brice Robin, on 26.03.2015 that Andreas Lubitz did »deliberately prevent the opening of the door« in order to deny access to the captain cannot be proven and is therefore purely speculative and was, in regards to the public statement, hastily stated by the prosecutor, Brice Robin.
Therefore, this assumption mentioned by Brice Robin is not based on any facts or evidence in the report.