Many customers were completely taken in by the fraudulent scheme.

taken 是动词 take 的过去分词形式，take 在这里表示"欺骗"的意思。 in 是介词，表示"进入某种状态"。 taken in 是一个固定短语，意思是"被骗、上当"，表示某人被欺骗或误导。