Just follow the marker located deep into Marshall. You are looking for a huge brick building with no entrances in the front. What you need to do is head at the back of the building and look for the fire escape ladder and jump over to grab it. Climb up the rooftop to find Ray. This will finish the mission.
After the conversation, survey your areas around Marshall and mark all activites you can find. There will be lots of Infestations here so what you need to do is clear out the Infestations first to avoid getting an alarming outbreak.