AG Eric Holder’s use of FBI planes is being questioned. (LARRY DOWNING/REUTERS)

In a letter sent Thursday to FBI Director Robert Mueller, the lawmakers questioned the propriety of the FBI footing the bill for Holder’s travel on FBI aircraft. “It is our understanding that the Justice Department does not reimburse the FBI, or other components, for its executive travel expenses even though the Justice Department maintains its own travel budget,” the letter states.

The letter was signed by Sen. Charles Grassley (R-Iowa), the top Republican on the Judiciary Committee; House Judiciary Chairman Rep. Lamar Smith (R-Tex.); and Sen. Kay Bailey Hutchison (R-Tex.) and Rep. Frank Wolf (R-Va.), the top Republicans on the relevant appropriations panels.

The Republican lawmakers say their inquiry was prompted by allegations that the FBI plane was being used for “extraneous business and personal travel by senior DOJ officials, including the Attorney General.” Grassley first raised questions about the DoJ’s use of FBI planes during a May hearing with Mueller.

An FBI spokeswoman said only that Mueller had received the letter and planned to respond to the questions. A DoJ spokesman did not have an immediate comment.

AGs are not permitted to fly commercial for security reasons, and like many government officials, they may use one of a pool of military jets maintained by the Defense Department, for both business and personal travel. They are required to reimburse the government, based on the cost of commercial fares, for personal trips.

The lawmakers said they were concerned that Holder’s use of FBI aircraft, which is supposed to be reserved for the agency’s own operations, could hinder the agency. Since the FBI always has to have a plane around — you know, in case they need to, say, sweep in and pick up an agent — the agency has had to lease another aircraft while theirs was being used to ferry DoJ officials, the lawmakers said.

“We heard troubling allegations that the Attorney General is among those who have reserved and used FBI planes for his own travel when aircraft were needed for FBI missions, then upgraded to a larger aircraft owned by a different agency and left the FBI plane sitting idle,” the lawmakers wrote. “These allegations were particularly troubling because they suggested the FBI had to lease another plane to ensure the availability of aircraft for FBI operations.”

During the May hearing, Mueller told Grassley that FBI planes “are used for counterterrorism and that any travel for principals is secondary to the use of the plane for the investigative work of the FBI.”

Congressional Republicans are at odds with Holder over a number of issues. The GOP-run House voted in June to hold him in civil and criminal contempt for withholding records on the Operation Fast and Furious gun-tracking effort.