The investigation was conducted in accordance with standard procedures.

in 是介词，表示"在...之中"或"按照"。 accordance 是名词，来自动词 accord（一致、符合），意思是"一致、符合"。 with 是介词，表示"与...一起"或"用..."。 in accordance with 是一个固定短语，意思是"按照、依照"，表示遵循某种规定或标准。