Letter from USCIS - What is additional review

二月份做了一个service request。然后今天收到回信：
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.

我现在除了这封信，什么也没有收到。有人知道这个是否表示他们要做local interview？还是一种标准回信方式？谢谢。