Doe or Die - S1.E17

Ken Rothman, an ex-police officer that Avery got fired for beating a suspect, gets into the police HQ by faking his death and he and his men quickly take the station and most of the staff (except Frank) hostage. Rothman demands his reputation be cleared, but John manages to get free and escape into the building, and soon discovers Rothman has a supply of Russian-made smallpox. John manages to take out a few terrorists before being recaptured and Rothman leaves John and Avery behind with a shattered vial of smallpox. It's fake, but the whole thing is a ploy to get hold of smallpox vaccine to sell as a bio-weapon. Doe figures it out and they capture Rothman just in time.