For the next two days, Nathan and Selena were hunting down as many aliens as possible.
Obviously, due to the attack on the two camps, they had to investigate where the aliens came from. As a result, they found the cave along with many corpses in it.
No one believed that fifty soldiers had been kidnapped into the cave and turned into aliens. And out of those fifty soldiers, only nine managed to become a host.
Nathan and Selena killed two of them.
Sullivan also led his soldiers to take down the remnant of those aliens, including five out of nine hosts. However, there were still two more hosts they couldn't find.
Even with Sullivan expanding the search area, the remaining two hosts simply vanished without a trace.
Hence, Sullivan had to change his approach, especially since those humans could be hidden among them.