Actually the pilot credited with shooting down von Richthofen was Roy Brown a Canadian, not American. And yes there is a good amount of evidence that von Richthofen was hit by a Vickers machine gunner on the ground about 600 yards away.
My favorite WWI ace would be the American Frank Luke of the 27th Pursuit Squadron, he flew a SPAD XIII, and was posthumously awarded the Medal of Honor. He is credited with shooting down 18 aircraft in 17 days, many of them balloons which were extremely hard and dangerous to shoot down because they were so heavily defended by anti-aircraft guns and airplanes.
"After having previously destroyed a number of enemy aircraft within 17 days he voluntarily started on a patrol after German observation balloons. Though pursued by 8 German planes which were protecting the enemy balloon line, he unhesitatingly attacked and shot down in flames 3 German balloons, being himself under heavy fire from ground batteries and the hostile planes. Severely wounded, he descended to within 50 meters of the ground, and flying at this low altitude near the town of Murvaux opened fire upon enemy troops, killing 6 and wounding as many more. Forced to make a landing and surrounded on all sides by the enemy, who called upon him to surrender, he drew his automatic pistol and defended himself gallantly until he fell dead from a wound in the chest." -Medal of Honor citation
"He was the most daring aviator and greatest fighter pilot of the entire war. His life is one of the brightest glories of our Air Service. He went on a rampage and shot down fourteen enemy aircraft, including ten balloons, in eight days. No other ace Britain's Bishop from Canada, France's Fonck or even the dreaded Richthofen had ever come close to that." -Edward Rickenbacker