Lockdown was the name given to an inactive state of operations in CTU. Lockdown is declared usually when there is a security breach within CTU, or when a mole among the members of staff is suspected or discovered.
A lockdown restricts access to the CTU computer systems and networks for all staff, and the CTU security teams block entrances to stop anyone from entering or leaving the building during the lockdown.
Members of the CTU team can be searched and checks on the system are carried out during security breaches. A lockdown is indicated by flashing red sirens, klaxon sounds and a freeze effect applied to the computer systems.
Division shut down CTU and declared a lockdown on "Day 1: 3:00am-4:00am." The lockdown was requested by Tony Almeida and done by George Mason in order to hold Jack Bauer. Jack, however, escaped to his car, assaulting an agent in the process. A further lockdown was carried out in the final hours of Day 1 when Nina Myers's cover was blown at CTU and she was suspected to be a security threat. During this lockdown, Nina put in to play a contingency plan and made contact with alliances of hers in Munich, Germany and deleted and took copies of her confidential files, before killing Teri Bauer when she discovered what Nina was doing.
A lockdown was put into effect when Gael Ortega attempted to escape CTU, after tying Kim Bauer up in Tech 1. He was stopped by Michelle Dessler and her guards. He was put into custody, only to be later revealed that he was in a sting operation with Jack Bauer and Tony Almeida involving Kyle Singer.
Later, in Season 3, Nina Myers escaped from the CTU medical clinic after backing her neck into a needle during her interrogation and Ryan Chappelle ordered a lockdown. She was found in an IT room by Kim Bauer and shot and killed by Jack Bauer.
Edgar Stiles discovered that Marianne Taylor was the mole inside CTU and that she framed Sarah Gavin. A lockdown was put into effect when Edgar told Erin Driscoll and Curtis Manning about Marianne and she tried to discreetly leave the building. Marianne got through the lockdown and escaped the building only to be caught by Curtis before she could get to her car., and was put into custody. Her car exploded after another agent put the key in to search it.
In the final moments of Day 4, Tony Almeida ordered a lockdown under the pretense of Jack attempting to escape custody; however, it was really part of a ruse to fake Jack's death in order to prevent Dale Spalding from killing him. The lockdown was canceled after Jack's 'death', allowing Tony and Michelle Dessler to sneak Jack out of CTU.
A lockdown was initiated when Ostroff managed to infiltrate CTU using Lynn McGill's keycard during Day 5 between 6:00pm and 7:00pm. However, the lockdown was converted to a Code 6 evacuation upon the discovery of a canister of Sentox Nerve Gas being introduced to the CTU ventilation system, and then was revised again when it was discovered that the gas was traveling from the perimeter inward, so that certain rooms were sealed to biohazard standards. Due to the circumstances surrounding the lockdown, evacuation, and eventual emergency sealing, roughly 40% of CTU staff perished.
A lockdown was put in place on "Day 6: 1:00am-2:00am" due to Jack breaking into the Clinic to prevent Audrey Raines from being shock-treated. As soon as Jack was apprehended, the lockdown was removed.
An hour later another lockdown took place because of troops sent by Cheng Zhi. They bombed a floor under CTU Los Angeles in the entrance area. Back in the center of CTU, Milo told Nadia Yassir to double security because they just lost all security cameras, after that Nadia frantically initiated a "code red emergency lockdown". Once they got in they killed at least 5 security guards, then took over the main floor, taking many CTU personnel hostage. After Milo said he was commanding officer of CTU he was killed. About 5 minutes into the attack, terrorists found Jack Bauer and Marilyn Bauer. The leader then went on the CTU PA threatening to kill Marilyn unless Josh Bauer showed himself to one of his men before he took her life. Josh complied and was captured. Shortly after that Jack, Morris O'Brian and Nadia executed a successful escape plan, ending the hostage situation. Mike Doyle then arrived to confirm that all hostiles were down, thus ending the lockdown.