Board Thread:General Discussion/@comment-90.228.250.139-20140922152434/@comment-29461586-20140922152839

The key to the trap door is carried by the Thalmor Agents who arrive with Malborn after you free Etienne and recover the dossier concerning Esbern.

After you kill the two Thalmor Agents who arrive, loot their bodies for the trap door key and make your escape.