question about the movie "the lovely bones".. how did the father realize who the killer was in the scene in?

his yard when he was building that fort out of sticks and mark
whalberg breaks his door. what tipped him off  made him realize that
he was the killer?
@lonely, yes i got that part. im talking about the part where whalberg
is in the mans back yard helping him build his fort  he all of a
sudden realizes hes the killer, gets angry  breaks his door. what
made whalberg realize he was the killer in that scene?