Find a passage to lower level | Chapter 11 - The Crash DI Riptide Guide

Return upstairs and approach the door, through which zombies were trying to get through. This time, zombies will succeed. Defeat Infested who will run at you and then use this door.

Move forward. Behind bars on right, you'll see new zombies - kill them, as you'll visit the area with them later on. Approach a new closed gate and examine it.