UFOs Over Chicago Airport

In January 2007, Chicago's Airport staff and a some pilots reported seeing UFOs hoovering over the airport. The FAA (Federal Aviation Administration of the USA) first denied receiving any reports of UFOs in that area but admitted receiving reports after freedom of information requests were submitted.

If there was indeed nothing that could not be explained, why does a government body try (once again) hiding information on UFOs?! For me, this is the real mystery.