Washington (CNN) -- The FBI said Friday it will respond to questions from Congress on the use of FBI aircraft amid allegations that Attorney General Eric Holder is among officials who "may have used FBI planes for his own travel when aircraft were needed for FBI operations."
Both the Justice Department and the Senate Judiciary Committee confirmed FBI Director Robert Mueller himself intends to reply to the allegations.
A letter to Mueller from key congressional Republicans in the House and Senate said Thursday that in one case, Holder is alleged to have reserved an FBI aircraft, "upgraded to a larger aircraft owned by a different agency and left the FBI plane sitting idle because he failed to notify the FBI in a timely manner."
The letter from Sens. Chuck Grassley, R-Iowa, and Kay Bailey Hutchison, R-Texas, was also signed by powerful GOP House Judiciary Committee Chairman Lamar Smith of Texas and House Appropriations Committee Chairman Frank Wolf of Virginia.
In May, Grassley asked Mueller, who was testifying on Capitol Hill, if the FBI had to lease a plane because the bureau's jet was reserved for top Justice Department officials.
Mueller responded: "The Attorney General, the Department of Justice, the FBI, understand that these planes are first for investigative work," Mueller said. "They are used for counterterrorism, criminal cases, and that any travel of the principals is secondary to the use of the plane for the investigative work of the FBI."
The answer was not sufficient for Grassley and his colleagues.
The lawmakers want to know why the FBI pays for Holder's travel, even when he uses non-FBI planes such as Department of Defense or Federal Aviation Administration aircraft.
Among several other demands, the lawmakers want the FBI to list all flights billed to the FBI by the Justice Department, and whether all of those flights were for official or personal travel.
The letter, apparently coincidentally, was sent to Mueller on the day The Washington Post highlighted what it called "Eric Holder's world tour." The Post noted that Holder is planning a trip to the Middle East. He has already traveled this summer on official business to Denmark, Germany, Guam, Malaysia and Singapore. The article notes the attorney general appears to have itineraries packed full of official meetings, so he has little time for sightseeing.
The expenses associated with the travel of attorneys general and other top government officials is an oft-debated topic in Washington. Frequently, the party out of power or its allies complain about the cost to taxpayers for official travel.
Holder's predecessor, Michael Mukasey, who served under President George W. Bush, was also questioned about his travel expenses.
One of the major issues is the government's requirement that for security reasons, attorneys general, Homeland Security secretaries, FBI directors, and many defense and civilian intelligence officials must fly in government aircraft.
When these officials travel on personal business they must reimburse the government at the rate of a round trip coach fare. The cost of a round trip coach fare between Washington and New York, for example costs about $130, while the actual cost of operating a government plane to and from New York to Washington is more than $4,000. The taxpayers pay the difference.