The L.A. County Sheriff's Department is the lead agency on Operation Independence, which is being run with the Department of Homeland Security. They are joined by officers with the LAPD and TSA.
The focus is on mass transit, specifically, the rail system that brings thousands of commuters in and out of downtown L.A. every day. From random bag checks to the presence of bomb-sniffing dogs, the drill was highly visible to the public.
The sheriff's department says the drill is not in response to a specific threat, and they are simply training for an event that would require a multi-agency response.
"Anytime you have a holiday, it's something that you want to look at," said Ted Sexton, chief of homeland security for the L.A. County Sheriff's Department. "The Boston bombing took place on Patriots Day; the Times Square incident took place on May Day; Benghazi on 9/11; so significant dates are always a concern to us."
Authorities are asking the public to remain vigilant during the holiday. The training drill will continue through Thursday.