So they gathered them and filled twelve baskets with the pieces of the five barley loaves left over by those who had eaten. 

left over 显然是过去分词。整个划线部分，作定语修饰 the pieces of the five barley loaves。后面有 by 引出动作的施事者。如果是名词的话，它在这里起什么作用呢？它跟前面的名词词组是什么关系，后面的 by 又怎么理解？