A nuclear weapon is a much sought after weapon for the agents of terrorism in 12. Even a nuclear weapon with a relatively small yield is significantly more powerful than the largest conventional explosives, and a single weapon can destroy an entire city. The panic and turmoil that would be caused by a terrorist detonating one of these weapons in a city such as San Francisco would be catastrophic, which explains why the FBI is so urgent to stop the threat.
At the start of Day 1, an anonymous call told FBI San Francisco that a nuclear weapon would be detonated in San Francisco that day. The FBI went into full recovery mode and did everything it could, including breaking into the Saudi Arabian Consulate in San Francisco.Once the FBI had found the bomb, they realized it could not be defused. While the detonation could not be stopped, it was decided that the detonation could be moved to an area with fewer casualties. A desert was chosen as the location, and Francis Newman volunteered to pilot the plane, as it would be too unspecific for it to be sent remotely. Francis Newman then died in the nuclear detonation.
Three hours later, the FBI had found out that the deceased Ronald Watson had a second nuclear weapon that was much smaller, but was still effective, somewhere in San Francisco. When the FBI found the location and assaulted on the building, the nuclear weapon was detonated and over 4,000 casualties were the price of this event.