After releasing the upper part, you need to detach the plastic key panels. Each of them is held by two latches inside the case.

To disconnect them gently, you can pry the key from the outside with a plastic card or a disassembly tool and use your fingers to gently press the latches from the inside. It is easier to do this with the right key than with the left key, because the latches of the left key are located deep inside the case.