I had gone for a walk from the city of Duarte (where I lived at the time, http://en.wikipedia.org/wiki/Duarte,_California) across the San Gabriel River (http://en.wikipedia.org/wiki/San_Gabriel_River_(California)) into the city of Azusa (http://en.wikipedia.org/wiki/Duarte,_California). On my return, as I neared the San Gabriel River bicycle path (http://en.wikipedia.org/wiki/San_Gabriel_River_bicycle_path) from the east, I heard and then saw five helicopters approaching from the southwest, flying in a slight and loose "V" formation. I stopped at the tree line that existed then in order to avoid being spotted; I don't remember my exact position, but I think it was about 34.14775 latitude, -117.925375 longitude. I don't remember the exact time, but I think it was between 9:30 PM and 10:00 PM PST. I presumed that they were U.S. Forest Service helicopters, which almost regularly swept the San Gabriel River from the Santa Fe Dam Recreation Area (http://en.wikipedia.org/wiki/Santa_Fe_Dam_Recreation_Area) to the top of the river's alluvial fan at night (and maybe still continue to do so). I immediately noticed that the helicopters were approaching an intensely bright ball of light about 15 to 20 feet in diameter; I didn't discern a structure or physical surface within it, but I immediately assumed that it was an alien spacecraft nonetheless. Each helicopter had a "headlight", and all of them activated at about this same time. The ball of light easily outshone the headlights. Being as bright as it was, I wondered how the ball had gotten to that position without me noticing it. The middle helicopter slowed to a stationary hover in "front" of the ball, while the other four flanked either side of it and rotated to face it. The helicopter personnel used the helicopters' public address systems to demand of the ball that it identify itself; this went on for a few minutes, but the ball did not respond to any of the demands. Then the helicopter personnel verbally inquired of each other how they should continue to handle this event; after a moment or so of discussion, the ball shot straight up out of sight in less than two seconds -- maybe in less then one second. It did not appear to accelerate gradually from motionlessness to a high speed, but instead seemed to immediately begin travelling at a high speed and maintain that same speed until it was out of view. The helicopter personnel expressed their amazement at what they witnessed, then mutually agreed that they would not report the event to anyone. The helicopters resumed their previous travel northeast along the San Gabriel River. I had remained more or less motionless for the duration of the event. I spent a minute or two getting over my own amazement, then crossed the river and returned home. I remember that I arrived at home sometime between 10:00 PM and 10:30 PM PST.I described this event to my father soon after I returned home, and he seemed to believe it. (When I was younger, he had told me of a UFO that he and his brothers encountered when they were young.)Over the years, I've told the story to a few other relatives and a few friends. I've been aware of MUFON since I was a teenager in the 1980s, but I never thought of telling my story to it. I described the aforementioned event to another relative earlier today, and he talked me into submitting it to MUFON.