'NCIS: LA' exec teases Callen mystery

NCIS: Los Angeles producer Shane Brennan has admitted that viewers will have to wait for answers relating to Callen's past.

It was revealed in the first season that a mysterious observer has been tracking the life of the special agent - played by Chris O'Donnell - since he was a child.

"The truth of what [Callen] finds out is startling," Brennan told TV Guide. "It's a mystery now and the further he goes down the rabbit hole, the deeper the mystery gets."

He explained that future episodes would instead shed light on Callen's boss Henrietta 'Hetty' Lange (Linda Hunt).

"There's a little moment at the very end of episode two that lifts the veil on Hetty," Brennan confirmed. "It's setting up something that comes later in the season that will be one of the most powerful episodes we do all season."

NCIS: Los Angeles will return to CBS on September 21.