NCIS: Los Angeles has not forgotten about Callen's mysterious past. The series will continue to explore the agent's hazy past throughout season 6.

Who Is Against The Densi Pairing On 'NCIS: LA'? One Agent Doesn't Support Deeks & Kensi's Relationship

Callen (Chris O'Donnell) has spent his entire life searching for clues about his family, and the drama's 100th episode doled out more information then ever before. Unfortunately, the fall season failed to deliver anymore backstory for the solitary agent.

However, just like NCIS: LA managed to work Joelle back into the mix, O'Donnell promises that there are plenty more secrets to uncover.

Callen vs Terrorists In Explosive 'Spiral'; 16 Story Drops & Elevator Shafts Promised On 'NCIS: LA'

"[Callen's past is] something they'll continue to explore and piece together what has happened," O'Donnell told Glamour. "For years, seeds have been planted, so it's time to start harvesting these things and exploring these story lines...the search for who he is will continue."

When NCIS: LA returns from winter hiatus, Callen spend "Spiral" trapped in a skyscraper overrun by terrorists. When Sam (LL Cool J), Deeks (Eric Christian Olsen), and Kensi (Daniela Ruah) come to his rescue, they will quickly realize that the entire building is rigged to explode.

Find out more when NCIS: LA airs Mondays at 10 p.m. ET on CBS. Click the video below to see a preview for "Humbug," the series' latest episode.