WASHINGTON - Members of Congress who travel abroad on the taxpayers’ dimes must file expense reports, but the largest cost of the trips remains a closely guarded secret.

The Examiner reviewed forms filed for dozens of such trips during the past two years and found that the cost of air travel is almost always excluded. In the boxes where that price is supposed to be listed, members instead write “Mil Air” to indicate that they flew on a fully staffed military jet.

“These costs are significant and often masked in a large Defense budget,” said Tom Fitton, president of the nonpartisan watchdog group Judicial Watch.

One of the only members of Congress who wants to reveal that information is Sen. Tom Coburn, R-Okla., who for years has badgered the U.S. Department of Defense and the State Department for the costs associated with traveling congressional delegations, known as CODELS.

A senior Defense Department official responded to one of Coburn’s requests by telling him the records are not kept. Coburn wrote back to then-Secretary of Defense Donald Rumsfeld, saying his department’s stonewalling surprised him, since Defense officials on Capitol Hill who arrange military transport for lawmakers are known to “take care of everything.”

“I am gravely concerned to hear that none of this activity on the part of the department is recorded, or financial expenditures tracked,” Coburn wrote to Rumsfeld. “I have found that taxpayers are quite interested in the accountability and transparency of every federal agency.”

Leaders of both parties take part in concealing the information, according to Coburn. Defense Department officials told him last year when his fellow Republicans controlled Congress that the majority leader and the chairman of the Senate Armed Services Committee instructed the department not to reveal the information.

“If members of Congress are so confident their trips are vital to their work,” Coburn told The Examiner on Monday, “they should disclose every detail about the trip in terms of itinerary, location, cost, sponsor and delegation members.”

One member of Congress who doesn’t share Coburn’s enthusiasm for full disclosure is Rep. Jim Oberstar, D-Minn., who chairs the House Transportation and Infrastructure Committee. He led a weeklong trip over Easter break to Brussels, Belgium, and Paris to “study mass transportation,” according to spokesman Jim Berard.

Asked who went on the trip, Berard told The Examiner that “it’s not the policy of the committee to disclose” the names of the travelling party, though he acknowledged that “some spouses” did go along.

Fitton denounced the common practice of taking spouses on taxpayer-funded trips.

“It gives the appearance that they’re not business trips but vacations,” Fitton said.

The Examiner reported Monday that more than a dozen congressional delegations traveled to Brussells, the Middle East and the Caribbean over the Easter recess.

Rep. Bennie Thompson, D-Miss., who chairs the House Homeland Security Committee, led a group through the Caribbean, spending one night at the pricey Caneel Bay resort in the U.S. Virgin Islands. Though room rates there go as high as $1,100 per night, a committee spokeswoman said the 20 rooms the delegation reserved each cost $220 per night.