WASHINGTON (AP) — Three Massachusetts lawmakers are pressing Defense Secretary Lloyd Austin to ground the V-22 Osprey aircraft again until the military can fix the root causes of multiple recent accidents, including a deadly crash in Japan.

In a letter sent to Austin on Thursday, Democratic Sens. Elizabeth Warren and Ed Markey and Rep. Richard Neal called the decision to return Ospreys to limited flight status “misguided.”

In March, Naval Air Systems Command said the aircraft had been approved to return to limited flight operations, but only with tight restrictions in place that currently keep it from doing some of the aircraft carrier, amphibious transport and special operations missions it was purchased for. The Osprey’s joint program office within the Pentagon has said those restrictions are likely to remain in place until mid-2025.

The Ospreys had been grounded military-wide for three months following a horrific crash in Japan in November that killed eight Air Force Special Operations Command service members.

There’s no other aircraft like the Osprey in the fleet. It is loved by pilots for its ability to fly fast to a target like an airplane and land on it like a helicopter. But the Osprey is aging faster than expected, and parts are failing in unexpected ways. Unlike other aircraft, its engines and proprotor blades rotate to a completely vertical position when operating in helicopter mode, a conversion that adds strain to those critical propulsion components. The Japan crash was the fourth fatal accident in two years, killing a total of 20 service members.

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Marine Corps Capt. Ross Reynolds, who was killed in a 2022 crash in Norway, and Air Force Staff Sgt. Jacob Galliher, who was killed in the November Japan crash, were from Massachusetts, the lawmakers said.

“The Department of Defense should be making service members’ safety a top priority,” the lawmakers said. “That means grounding the V-22 until the root cause of the aircraft’s many accidents is identified and permanent fixes are put in place.”

The lawmakers’ letter, which was accompanied by a long list of safety questions about the aircraft, is among many formal queries into the V-22 program. There are multiple ongoing investigations by Congress and internal reviews of the program by the Naval Air Systems Command and the Air Force.

The Pentagon did not immediately confirm on Friday whether it was in receipt of the letter.