Ostroff

Ostroff was a terrorist working for Vladimir Bierko during Day 5. He led a failed attack on Yuri Suvarov's motorcade and carried out a successful nerve gas attack on CTU that killed 56 people, including Edgar Stiles.

As part of the attack on CTU, Ostroff arranged to purchase Lynn McGill's key card from Dwayne Thompkins for $20,000. It is unclear whether Ostroff contacted Dwayne and instructed him to steal the card, or whether Dwayne stole the card and arranged the sale later.

After shooting Dwayne and Jenny McGill, Ostroff took the key card and reprogrammed it with his photo and the name "Mark L. Marcus." The reprogrammed card let him into CTU, where he used the security codes on the card to block remote access to the ventilation system. Carrie Bendis interrupted him while he was setting up the cannister of nerve gas, and he stabbed her to death.

Ostroff's escape from CTU was hindered by a lockdown of the building ordered by Bill Buchanan. Ostroff killed a security guard on his way out, but was in turn shot and killed by Jack Bauer.