上个月发了一个I485的service request，今天收到了回信：
Based on your request we researched the status of this case. We are actively processing this case. However, we have to perform additional review on this case and this has caused a longer processing time. If you do not receive a decision or other notice of action from us within 6 months of this letter, please call customer service at the number provided below.
想请问一下在什么情况下，会需要additional review？多谢！