parent's green card

Hi, 
my parents just had green card interview with uscis. everything went ok but one additional requirement. they asked us to go city police office station to get police clearance letter for my parents, we went to city police dept and they told us they dont do that type of work, everything is handle by the county police dept. they direct us to county police dept where we went and get background form filled and stamped. my question is criminal background check is the same as police clearance letter? does uscis is willing to accept it even from county police dept instead of city police dept.