Hi,
I got the letter today
" Processing Delay
This refers to the application(s) for a Permanent Resident Card that you filed with this office. The purpose of this letter is to advise you that a preliminary review of your application has been completed. You r file has now been referred for a secondary review. This review will result in an additional delay. However, we will endeavor to finalize your application as soon as possible. You will be contacted if additional information or documents are required to complete your file."
Although my status shows the letter sent out on January 31 but mailing date was Feb 03 :