One pilot reportedly pushed the plane sharply downward, while the other pilot fought him and managed to pull the plane back up.
The plane fell 14,125 feet in just 29 seconds.
A passenger said they heard shouting. When the cockpit door opened, there was blood inside, and one of the pilots had reportedly been hit with a folding knife.
Passengers and crew members stepped in to stop the fight. At one point, people on the plane thought they might not survive.
The captain is reportedly Russian, while the first officer is Ukrainian