The Pentagon’s newly installed top weapons buyer told a Senate panel Thursday that she was “determined to reduce the cost” of the F-35 Joint Strike Fighter, a price tag projected to top $1 trillion over its 60-year life span, making it the most expensive weapons system in the history of the Defense Department.

Speaking before the Senate Armed Services Committee, Ellen Lord, the undersecretary of defense for acquisition, technology and logistics, said the Pentagon would work with the program’s prime contractors, Lockheed Martin, Northrop Grumman and BAE Systems, to “improve cost performance.”

Shortly before he was inaugurated, President Trump took aim at the F-35, saying the “cost is out of control.” He vowed that once his administration took office, it would save “billions of dollars.”

Since then, he’s softened his stance, recently lauding the “awesome power and stunning beauty of the F-35” during a visit to Joint Base Andrews in Maryland. “When our enemies hear the F-35 engines, when they’re roaring overhead, their souls will tremble, and they will know the day of reckoning has arrived,” he said in September.

In a statement, Lockheed Martin said it has reduced costs since the first production lot and would work with the government to identify new cost savings.

“We continuously implement projects across our development, production and sustainment operations, as well as within our supply chain, to drive down costs,” the company said.

Thursday's Senate hearing came a few weeks after Congress’s watchdog agency took aim at the stealthy fighter jet's “sustainment costs,” those associated with maintaining and operating the fleet, which are expected to reach $1.1 trillion over the life of the program.

The Pentagon’s ability to repair F-35 parts at military depots are six years behind schedule, “which has resulted in average part repair times of 172 days,” according to a report recently released by the Government Accountability Office. That timeline is twice as long as the goal for the program.

Further, the report found that “spare parts shortages are degrading readiness.” From January to August of this year, F-35s were unable to fly 22 percent of the time “due to parts shortages,” the GAO said.

While sustainment costs have risen as the Pentagon has increased flying hours and extended the life of the program, those costs “are not fully transparent to the military services,” the GAO said.

Joe DellaVedova, a spokesman for the Pentagon's F-35 Joint Program Office, said that driving down the cost is the "number one overarching priority." The office has established an "Affordability War Room" that creates "a single location for experts in manufacturing, supply chain management, cost estimating and acquisition to work jointly on driving down [operation and sustainment] costs," he said in an email.

In the past four years, that effort has lowered estimated costs by more than $60 billion, he said, and should reduce it by another $110 billion by 2023.

The struggles with the cost come as production by manufacturer Lockheed Martin is about to ramp up. Currently, there are some 250 F-35s in operation, the report said. That is slated to triple by the end of 2021 and eventually could grow to as many as 3,200 aircraft to countries around the world.

The report also notes the complexities in keeping a jet as advanced as the F-35 up and running. The F-35 comes in three variants, one each for the Air Force, the Navy and the Marines, and is designed to eventually replace the military’s current fleet of fighter jets. It is also being sold to several foreign militaries. Known as a fifth-generation fighter, it has many advanced capabilities, such as its radar-avoiding stealth. It’s often called a flying computer, and with 8 million lines of code, it serves as a flying sensor in the air as much as an attack aircraft.

For years, Pentagon officials have been adamant that the problems plaguing the program — billions of dollars in cost overruns and years of delays caused by technical difficulties — were behind them. As they declared that the program was finally on track, the program achieved several important milestones: In 2015, the Marine Corps declared its version of the fighter was ready for combat. Then the Air Force did the same last year.

But as the rate of production climbs, the high-profile program will continue to attract scrutiny. “Right now, an enormous amount of my time is focused on F-35,” Lord told the Senate panel.