He hides the old man's body under one of the wooden planks/floor boards. However, he later goes insane and imagines that he can hear the heart still beating and digs the body out. It's been a while since I read it so I might be wrong, but I'm pretty sure I'm right.

Yeah mostly right so two police guys come over and his insanity begins to take over so he thinks that they KNOW he has the body under the floor (they don’t) so he completely loses it and tells them that they can stop pretending. He then tells them where the body is .

Explanation:

the decreasing neutrality of the u. s. in world war i

hope this : )