Both Uses of

culprit
in
A Prayer for Owen Meany

That day at Hurd's, you could almost hear those boots creaking by the door, as if he expected the stolen baseball to loose itself from the culprit's pocket and roll across the dark crimson carpeting with incriminating authority For Chief Pike, the theft of the ball that killed my mother was an offense of a far graver character than a mere misdemeanor; at the very least, it was the work of a felon.†
p. 134.3

culprit = person responsible for a wrongdoing

As always, he was stationed by the door, his suspicious eyes roaming the audience as much as they toured the stage, as if Chief Pike suspected that the culprit might have brought the stolen baseball to the play!†
p. 241.7 *

Definitions:

(1) (culprit)
someone responsible for a wrongdoing -- especially a criminal; or something that caused a problem
(2) (meaning too rare to warrant focus)
meaning too rare to warrant focus:

More rarely culprit can reference anything identified as the cause of a problem -- such as a defective part that broke a machine.