The exercise that began Monday involves a team of state, local and national agencies dealing with an improvised nuclear bomb planted at the landmark Los Angeles Coliseum.
"Threats involving weapons of mass destruction are complex, and confronting them must be practiced constantly," Assistant Director Steve Martinez, the head of the FBI office in Los Angeles, said.
The FBI says in a written statement the scenario involves finding not only the bomb, but also secondary devices in Los Angeles. Once located, the "bomb" will be disabled and hauled away to a safe location.
A plot twist includes the detonation of another device in another pat of the United States, the FBI said.