U.S. safety regulators said they have expanded to 2.7 million vehicles an investigation into the possibility that straps holding fuel tanks can rust and break on Ford Motor Co F-150 pickup trucks from the 1997-2001 model years.

The U.S. National Highway Traffic Safety Administration said over the weekend that two fires have resulted from the issue in which one or both of the steel straps that hold the F-150 fuel tank in place rust and break.

An investigation is short of a recall but many investigations lead to an automaker recalling vehicles for safety issues.

The F-150 is the primary model in the Ford F-series pickup truck lineup which are the best-selling vehicles in North America.

We are working with the government as they review the matter, said Ford spokesman Wes Sherwood.

Ford reported a complaint in which a vehicle was destroyed by fire due to a leaking fuel tank, and safety regulators said they had received one complaint of a fire that went out before it caused major damage.

There were 306 consumer complaints, 175 filed with the NHTSA and 156 with Ford. Some consumers filed with both the automaker and the safety regulator, the NHTSA said.

Among the incidents reported to NHTSA or Ford, 243 involved the fuel tank dropping below the vehicle and/or dragging on the ground, 95 involved fuel leakage, and nine included reports of sparks from the tank being dragged on the road, the NHTSA filing said.

The probe was opened last September when an estimated 1.4 million F-150 pickup trucks were suspected of possibly having the problem. The investigation was opened after 32 consumer complaints had been received by last September.

(Reporting by Bernie Woodall, editing by Dave Zimmerman)