WASHINGTON — The agency that tracks federal travel did not report hundreds of expensive personal trips aboard government planes for senior Justice Department officials including Attorney General Eric Holder and former FBI Director Robert Mueller, according to a watchdog report.

Congress’s nonpartisan Government Accountability Office, or GAO, determined that the 395 flights cost taxpayers $7.8 million. But the General Services Administration, or GSA, which oversees such trips, did not require documentation because of a GSA reporting exemption that covers intelligence agencies, even in cases of unclassified personal travel.

The GSA exemption contradicts decades-old executive-branch requirements, specifically guidelines established by President Bill Clinton and the Office of Management and Budget, according to the report. The report said GSA “has not provided a basis for deviating from executive branch requirements.”

The findings, released Thursday, came out nearly 19 months after Republican lawmakers began questioning Holder’s use of an FBI jet for travel unrelated to Justice Department work. Sen. Charles Grassley, R-Iowa, the ranking member of the Senate Judiciary Committee, asked the GAO to look into the matter.

“Secrecy of personal or nonmission trips taken at taxpayer expense only serves to create a distrust of the federal government,” Grassley said in a statement on Thursday. “The GAO identified a significant gap that needs to be addressed to ensure transparency and verify that federal agencies are following current regulations.”

For security reasons, attorneys general are required to use non-commercial flights when they fly, and they have access to Defense Department jets. However, they must reimburse the government for personal trips.

According to the report, GSA has promised to eliminate the exemption that applies to nonmission, unclassified travel for intelligence officials. The agency also plans to begin documenting those trips in its periodic reports on travel by senior federal officials.

Justice Department spokesman Brian Fallon noted that DOJ makes its travel information available to Congress and members of the public when they request the information under public-records laws.

“If GSA intends to also report this information regarding unclassified, non-mission travel, we would have no objection at all,” Fallon said.