Source? I am about 5 miles from San Dimas, didn't hear a thing. Unless it just crash landed.
I'm in the area as well. Grew up swimming at Puddingstone. KNBC-4 had this starting around 12:45. Looks like a Cessna 310 went down in a crowd of people. There appears to be no fire, which is unusual if this was a fully fueled Cessna coming out of Brackett Airport for a long flight. KNBC reports one dead and two children in cardiac arrest.
Maybe it was an accident. Maybe not.