The Los Angeles County Sheriff's Department said it seemed to be a routine transport of military helicopters from point A to point B, but no further details were available.
Eyewitness News viewers in downtown L.A., Compton and Pasadena reported seeing about 13 military helicopters fly overhead.
"Scared the bejeezus out of us in #Pasadena our house & windows were RUMBLING and RATTLING! phew!," Nicole Biegenzahn Tweeted.
The military choppers were seen practicing maneuvers in downtown L.A. about two weeks ago.