Find a way into the Channel 13 building
Before heading up to the Channel 13 news building, go over to right and grab the Scrap from the blue van, now climb up to the raised area and use the rope hanging out of the window to get inside. Follow the path down and go into the side room to find some Supplements, before crawling through the vent.
Once you've dropped down to the area of hanging bodies, go into the booth on the left to find some Scrap, now go upstairs and walk along the edge to reach an office. Head to the office at the far back to find a Trading Card on the desk, then go through the door to the right of the body to head go up to the next floor. Here you'll want to explore the office on the left first to find an Artefact on the chair.
Finally, go into the next room to find some Supplements on the desk, then go into the room at the end to view a scene.
Escape the building
Upon discovering Leah's fate, you'll need to escape the building full of WLF Soldiers. In order to do this you'll want to drop out the window ahead and then go down the stairs into the room full of hanging bodies, now use the cover around you to ignore the enemies and head for the open door at the back of the room.
Once you've made it, there will be a few more enemies waiting for you above, so just sprint ahead until you reach the tunnels.