Plutonium

Plutonium, a radiological material, was the cause &mdash; albeit indirectly &mdash; of George Mason's death during Day 2.

Day 2
The nuclear bomb that terrorists assembled on Day 2 was made with plutonium. When LAPD found the garage where the bomb had been constructed, they investigated with the help of George Mason. When they arrived, some of the terrorists were still there. Although the LAPD managed to kill them all, the terrorists shot the glass covered plutonium case, releasing the toxin. Mason was stood in close proximity of the case, and was exposed to the radiological toxin.

Based on the particle size, atmospheric saturation, and length of time exposed, the exposure level was lethal. He had between one day and a week to live.