1. Hurk's third letter
Follow the path to reach the area marked as number one. In the designated place, there is a house, next to which there is a wooden structure to the right (the screenshot). Next to the structure, there is a manhole with a ladder.
Once down there, reach the other end of the foxhole to find a corpse and the letter.
2. Hurk's first letter
In the area marked as number two, you will find remains of a bunker shown in the screenshot. Enter the structure, where there is a corpse and the letter in the corner to the right.