The Air Force is still considering retiring many of its fourth-generation F-15 Eagle fighters — and it could happen by the end of the next decade.

Air Force officials said in March 2017 that it was considering retiring its 236 F-15 C and D fighters and replacing them with F-16 Fighting Falcons.

Lt. Gen. Jerry Harris, the Air Force’s deputy chief of staff for strategic plans and requirements, told Sen. Elizabeth Warren, D-Mass., during a Senate Armed Services air land forces subcommittee hearing Wednesday that the service is still studying its options with the F-15 ― especially as it brings the newest F-35 fighters on board.

“There’s nothing off the table,” Harris said. “We’re looking at, as we bring F-35s in, can we grow our capacity rather than just replace one-for-one? If we can’t do that, what’s our least-capable asset to retire, based on the value that it would provide for us?”

Harris said it remains unclear whether the F-15 will last until the end of the 2020s, and that it’s also looking at other older aircraft as well.

“We’re looking at our F-16s, our A-10s, and our F-15s to make sure that we have the best, capable Air Force,” Harris said. “The F-35, as a multi-role airplane, is very capable of some of the things the F-15C, the F-16 and the A-10 do.”

Lt. Gen. Arnold Bunch, the military deputy for the Office of the Assistant Secretary of the Air Force for acquisition, said that the Air Force is continuing to conduct tests on potentially rolling the new Eagle Passive/Active Warning and Survivability System defensive program to the F-15C, as well as the F-15E.

However, the Air Force is holding off on actually installing the EPAWSS on the F-15C until it decides whether it will keep it, Bunch said. He could not give Warren a potential timeline on that decision.

“We have not closed the door on that, that [testing] is something we are doing to make sure we have that flexibility,” Bunch said.

Bunch said the Air Force will keep the F-15E Strike Eagle fleet, and is concentrating its fiscal 2019 funding on paying for upgrades to those fighters.