The decision to buy new Boeing F-15s reflects the Pentagon’s desire to keep two American companies making fighter jets into the next decade — and not the acting defense secretary’s ties to the company, a senior defense official said Friday.

The 2020 budget request contains $1.1 billion to buy eight F-15X jets, a new variant of an aircraft the Air Force last bought nearly a decade ago. The twin-tailed plane was chosen over Lockheed’s cheaper single-engine F-16 in part to keep a second U.S. manufacturer in the tactical-jet business as the Pentagon begins exploring new technologies for a new generation of warplanes, the official said.

“One of the considerations was the diversity of the industrial base,” the official said. “If we look at something as important as the tactical aircraft industrial base and we look forward into sixth-generation [fighter] production and competition and that kind of stuff,…gaining diversity in that industrial base is going to be critical.”

The senior defense official emphasized that Acting Defense Secretary Patrick Shanahan, who formerly worked as a Boeing executive, was not involved in the decision to buy the F-15X.

“When it came to any specific platform that involved Boeing, those conversations were held strictly away from him,” the official said.

Upon entering government service in 2017, Shanahan formally recused himself from all dealings involving his former employer. But on Wednesday, the Defense Department’s inspector general confirmed that it had opened an investigation into “complaints that we recently received that Acting Secretary Shanahan allegedly took actions to promote his former employer, Boeing, and disparage its competitors, allegedly in violation of ethics rules,” an IG spokesman said. On Friday, the Pentagon released an updated ethics agreement that Shanahan signed when he became acting secretary in January.

Since Shanahan and other top Trump administration political appointees at the Pentagon have ties to the defense industry, their aides are trained to make sure the officials are not involved in decisions related to their former employers, a second senior defense official said. They were extra cautious, particularly during budget deliberations.

“We had to be very careful about it,” the second official said.

Last fall, the Pentagon’s Cost Assessment and Program Evaluation office recommended that the Air Force buy a mix of F-35s and an in-production fourth-generation fighter — a candidate pool that includes the F-16, F-15, and Boeing’s F/A-18. By mid-October, the Air Force agreed, and picked the F-15. Then-Defense Secretary James Mattis approved the plan, Pentagon deputy controller Elaine McCusker said last week.

Shanahan’s “part of that process was really limited to broad capability discussions and force-shape discussions, but it really didn’t get down into any of the specific platforms,” the first official said.

The Air Force, Navy, and Marine Corps, as well as a growing number of U.S. allies, are buying the fifth-generation F-35, a stealthy warplane with advanced sensors that Pentagon officials say is critical to the wars of the future. For years, the Air Force has refused to buy new F-15s or F-16s even as their existing fleets grew tired from nearly two decades of use supporting ground troops in Iraq and Afghanistan, opting only for the F-35.

But the senior defense official said Friday that fourth-generation planes, particularly the F-15 has an essential role in the future battles, signaling out homeland defense a protecting U.S. bases in the Pacific.

“We can do those other missions with fourth-[generation] planes more affordably,” the official said. “Our fourth-gen inventory as it is right now is aging out and we’re starting to see some capacity shortfalls.”

The senior officials said that fourth-generation aircraft will remain useful in counterrorism operations, even as the Pentagon’s National Defense Strategy focuses on near-peer fights with Russia and China.

F-15E Strike Eagles — an older version of the F-15EX — have been used on and off for close air support missions in Afghanistan, Iraq, and Syria for nearly 20 years. The officials said that the F-15s, when armed with long-range cruise missiles, could also play a role in a high-end battle.

“There is a role for penetrating assets [like F-35] to go downtown, to get inside their air defenses,” the official said. “But there’s a role for standoff munitions.”

The decision to buy the F-15s, a move that Congress must still approve, puts pressure on Lockheed to lower the cost of its F-35 Joint Strike Fighter, which for years experienced of developmental delays and cost overruns. The price of the F-35 has been falling in recent years and the reliability of newer jets improving.

The first senior defense official said the Pentagon expects the new F-15EX to cost about $90 million per copy. The F-35 is expected to cost about $80 million in 2020.

Lockheed has argued that its F-35 would cost the Pentagon less than new F-15s, and that it could increase production to meet demand.

“Should the U.S. Air Force or any other customer plan to increase their annual procurement rate, we are confident we can meet increased demand — while continuing to deliver an $80 million F-35A by 2020, which is equal to or less cost than legacy aircraft,” Michael Friedman, a company spokesman, said in an email Friday.

Right now, the Air Force plans to buy 240 F-35s between 2020 and 2024.

“Lockheed Martin and our partners have been ramping our manpower, material, methods and machines to ensure we are prepared to deliver the maximum number of aircraft across the three Final Assembly and Checkout facilities for our current and growing customer base,” Friedman said. “With these actions, the F-35 enterprise has capacity to deliver about 430 U.S. Air Force F-35As in that same timeframe, or about 190 more than currently planned.”