Federal regulators have ordered Chrysler to document why the automaker is taking so long to repair rear fuel tanks in Jeep SUVs, Reuters reported.

At the current pace of the recall, Chrysler will spend most of the next five years to fix the 1.56 million Jeep Grand Cherokee and Jeep Liberty vehicles affected, according to the National Highway Traffic Safety Administration.

The agency has issued a "special order" that requires Chrysler to hand over documents explaining why repairs have been so slow.

The June 2013 recall included Jeep Grand Cherokee for model years 1993 to 1998 and Jeep Liberty from 2002 to 2007.

According to Chrysler, suppliers are working six days a week to produce the trailer hitches needed for the repair. At the time of the recall, the automaker said that trailer hitch assembly placed on the vehicles would shield occupants during a low- or medium-speed rear-end collision.

Chrysler agreed to make the recall last June, but the carmaker didn't select a supplier until December 2013 or order the trailer hitches for production until late January.

In response, Chrysler issued this statement: "To accommodate the high-volume production required for this campaign, Chrysler Group had to find and enlist multiple new supplier partners to supply volume of this part that far exceeded normal demand."

In the special order from the NHTSA to Chrysler, the agency's chief counsel, O. Kevin Vincent, wrote, "For many owners, a recall remedy deferred by parts availability easily becomes a defect remedy denied."

The NHTSA formally requested that Chrysler recall the vehicles last year after reports of gasoline fires following rear-end crashes, The Detroit News reported. The faulty gas tanks have been related to 51 deaths, according to government regulators. 

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