Sometimes they were the size of a basketball and sometimes much bigger. American piots at that time knew rockets had been perfected by the Germans and many saw UFO's - knowing they too were being produced by the German's. The foo fighters would get in formation with a bomber and would stay. They couldn't shoot them down or out maneuver them. Some of them would come as close as 10 feet to the American's airplane cockpit.
Some people feel they still exist.
Stephen J. Brickner, a sergeant with the 1st Marine Division, had an encounter with mysterious aerial objects. It was a bright tropical morning with high banks of white, fleecy clouds. I was cleaning my rifle on the edge of my foxhole, when suddenly the air raid warning was sounded. I immediately slid into my foxhole, with my back to the ground and my face turned up to the sky. I heard the formation before I saw it.