|Alias(es):||Mark L. Marcum|
|Death date:||Day 5: 6:56pm|
|Death cause:||Shot by Jack Bauer|
|Played by:||Alex Kuznetsov|
|First episode:||"Day 5: 3:00pm-4:00pm"|
|Last episode:||"Day 5: 6:00pm-7:00pm"|
Ostroff was a Dawn Brigade 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.
After the deaths of a number of Dawn Brigade separatists earlier in the day, Vladimir Bierko secured many additional men and resources at great personal expense. Ostroff was the primary terrorist among this later group.
When Bierko realized that James Nathanson betrayed Ivan Erwich, a mutual associate, Bierko ordered for Ostroff and his men to handle the situation. Ostroff was first seen inside the dark computer lab where Nathanson had coordinated much of the earlier parts of the conspiracy. There, he executed Nathanson's subordinate, and set out to find his primary target. One of Ostroff's men then killed Alpha 7, a fellow conspirator of Nathanson's. While driving away, Nathanson heard Alpha 7 being shot over the phone, and then contacted Jack Bauer to cut a deal.
Ostroff's men tracked down Nathanson as Jack approached his location. Bauer observed several hostiles exit from a helicopter, and warned Nathanson, who managed to escape to the roof. After a running firefight, shooters in the helicopter gunned down Nathanson. Jack returned fire and disabled the chopper, which then made an emergency landing. Ostroff informed Bierko that Nathanson may have passed over information to someone before dying.
When Bierko manage to coerce President Charles Logan into giving up the motorcade route of Russian President Yuri Suvarov and his wife Anya, Ostroff led the terrorists that was ordered to take out the motorcade. Unfortunately for Bierko, CTU Los Angeles learned about the attack and warned Secret Service. Though Ostroff's men killed nearly all of the surrounding agents, Aaron Pierce shot the last of them.
Bierko then ordered for Ostroff to gain access to a CTU keycard, in order to carry out one of the cell's biggest attacks. Ostroff staged a black market purchase of Lynn McGill's CTU keycard from Dwayne Thompkins and Jenny McGill, but murdered both of them instead of paying. He then told Bierko it would only take 5 minutes to reprogram it, and he would be in CTU within the hour.
After gaining access to CTU, Ostroff programmed a canister of Sentox VX nerve gas to open inside a ventilation shaft on a timed countdown. When Lynn McGill learned about the death of his sister, he informed Bill Buchanan about his stolen keycard, and Chloe O'Brian checked the logs. When Chloe learned that someone used Lynn's card to access CTU, Buchanan ordered for her to initiate a lockdown.
After taking a guard hostage, Ostroff killed him when he realized he would be no help, and grabbed his radio. Jack managed to trick Ostroff into thinking all the agents were on the roof looking for him, which lured Ostroff out into the open. After Jack found Ostroff, Jack demanded for him to drop his weapon. Ostroff complied, but Jack killed him by shooting him twice, when he reached for a second firearm.
Despite CTU being functional for the rest of the day, Ostroff's attack on CTU was partially successful. The nerve gas killed roughly 40% of the staff, including Lynn McGill, Edgar Stiles, and Harry Swinton.