The Freedom of Information Act (FOIA) 

The U.S. Freedom of Information Act (FOIA) is a law ensuring public access to U.S. government records. FOIA has a disclosure presumption; the burden is not on the public but on the government - to verify why data may not be released. Upon written request, agencies of the United States government have to release those records, except when they can be legitimately withdrawn from disclosure under one of nine precise exemptions in the FOIA. This right of access is eventually enforceable in national court. 

Can read more about the act here: