Senior federal officials use government airplanes for many non-official purposes, but agencies don't track the trips, according to the Government Accountability Office.

More than 1,700 aircraft are owned or leased by the federal government and are used to "accomplish a wide variety of missions," including firefighting and law enforcement, according to GAO.

After a February 2013 GAO report criticized how the General Services Administration told the Department of Justice to report travel, GAO began a more detailed audit this year into GSA oversight of senior federal officials' travel.

When the aircraft are used for other purposes, the officials are supposed to report such trips every six months to the GSA.

The problem is, according to GAO, that "GSA exempted intelligence agencies from reporting any information on senior federal travel on government aircraft regardless of whether it is classified or unclassified."

According to GSA officials, the agency doesn't put forth travel regulations because of "conflicts" they may cause with other agencies' rules.

GSA also can't say which agencies report the travel when they do, thus further limiting the accuracy of the reports.

From fiscals years 2009 through 2011, for example, the FBI didn't report to GSA 395 unclassified non-mission trips that cost around $7.8 million, according to GAO.