Sarah Mason was a CIA deep-cover agent, participant in Project Red Bell, and lover of her teammate Robert Hawkins. Sarah was a traitor to the team, loyal to those responsible for the September Attacks. Sarah pretended to be kidnapped by a group seeking the device as ransom, but the nuclear attacks happened before anything could be discussed.
Following the Attacks and the deaths of the other members of Red Bell, Sarah comes to Jericho as a refugee, and re-connects with Robert, planning to obtain the 20-kiloton nuclear device in his possession and keep it for herself, double-crossing her employers. Sarah initially gains Robert's trust, but he comes to suspect her, so she seizes his son Samuel as a hostage, whereupon she is killed by Allison Hawkins.
In Season 2, the ASA, unaware of Sarah's death, declares her a terrorist to be shot on sight, alleging that she had a hand in the Attacks, in order to suppress what she knows. Major Beck leads the hunt for her (and his failure in this damages his position) until "John Smith" calls Beck and reveals enough truth to frame Hawkins as a terrorist and (temporarily) cost him the bomb.