@arthi85, when we sent expedite request through congress, USCIS emailed to congress a email saying we need a valid reasons to expedite such as 1. medical, 2. financial loss, 3. government relations or 4. USCIS error. It is a standard letter from USCIS to Congress which was forwarded to us from Congress.
Did you get a letter directly from USCIS in addition to email to congress? it is little confusing. so, trying to understand. And again, this could be random and different for each applicant. Let's pray for good things to happen.