The police said the accident could have been caused by ice on the road.

could 是情态动词 can 的过去式，在这里表示"可能性"。 have 是助动词，与 been 构成完成时态，表示"已经"或"曾经"。 could have been 是一个固定结构，表示"本来可能/也许已经"，用来推测过去可能发生但不确定的事情。