From the Los Angeles Fire Department news blog:
This morning, more than a dozen public agencies in the Greater Los Angeles area will be participating in "Operation Safe Passage", a disaster preparedness drill.
"Operation Safe Passage" is a functional training exercise that simulates terrorist attacks at regional airports.
This off-site "virtual" training for public officials has no impact whatsoever on air travelers or aviation facilities.
As a result of this drill, public safety agencies may broadcast realistic sounding radio messages that simulate the events of a real emergency. We wish to emphasize that this morning's event is merely training.
Again, there are no airport emergencies currently in progress in the Greater Los Angeles area. Local government agencies will be conducting this drill until noon local time.