@scifi
Not sure of your interpretation of the article here. It says
"Four agents of national police were traveling on the flight and as mark the protocol in such cases, the captain requested them to give up their weapons to be able to keep them in the cabin until they reached Madrid.
To prevent passengers to see the weapons, police turned away to one side of the row and manipulated the weapons inside a suitcase. However, one shot was fired but the bullet never came out of the suitcase."
To me, this suggests that the agents were unloading their weapons (removing the magazine and working [by hand] the mechanism several times to ensure there are no rounds in the chamber following which they would pull the trigger to reset the firing mechanism). This reads as if it is a classic accidental discharge during the unload, possibly caused because they were probably in a rush and doing it in a concealed way.
Thankfully, no one was hurt, but the question for me is why wasn;t this done BEFORE they got on the aircraft?