WASHINGTON (AP) — The military's hundreds of V-22 Ospreys will not be permitted to fly their full range of missions until at least 2025 as the Pentagon continues to address safety concerns in the fleet, the head of the program told lawmakers at a hearing Wednesday.

Vice Adm. Carl Chebi, the head of U.S. Naval Air Systems Command, which has responsibility for the V-22 military-wide, told lawmakers at a House oversight hearing into the program's recent crashes that it will be at least another six to nine months before the command will be able to complete all of the safety and performance assessments for the Osprey.

Over the lifespan of the program, Chebi said a total of 64 service members have been killed in air and ground accidents, and 93 have been injured.

But over the last two years, four separate crashes killed a total of 20 service members and two of those crashes involved catastrophic materiel or mechanical failures the program had not experienced before.

Following a November crash off the coast of Japan that killed eight servicemembers, the fleet was grounded for months. The Ospreys were returned to flight status in March, but in a very limited format, and do not perform the full range of missions, including carrier operations, that the aircraft was made to carry out.

In use since only 2007, the Osprey can fly like an airplane and land like a helicopter. Critics of the aircraft say its innovative design has systemic flaws that are driving the unexpected failures.

One of the reasons for the extension of restricted flight: The military is still working to fix a clutch failure that was identified as one of the primary factors in a June 2022 crash that killed five Marines in California.

The clutch component, like many other parts of the vehicle, has been wearing out far faster than expected. This led to an unprecedented dual hard clutch engagement in the 2022 crash, creating a situation in which the pilots had no way to save the aircraft.

The military has not yet said what exact part failed in the November crash, but Chebi told the panel Wednesday that the cause was something "we'd never seen before."

Massachusetts Democrat Rep. Stephen Lynch told Chebi to reground the entire fleet until all safety issues were fixed.

"What do you think the consequences will be if we have another V-22 go down and we lose more brave Marines or Airmen between now and the time ... that we might have this clutch situation figured out? Your whole program's done. It's done. If another Osprey goes down, we're done. This program's done. So why don't we ground this now?"

The committee is looking into whether the program has adequate oversight, but to date has not received the data and documents it has requested on the program, members said at Wednesday's hearing.

Among the information that the House Oversight Committee's Subcommittee on National Security, the Border and Foreign Affairs has requested but has yet to receive is the wear and replacement rates on Osprey proprotor gearboxes, a component that was a factor in the crash off Japan.

Committee members also have asked for internal crash reports that the military conducts with surviving air and ground crews and witnesses. The reports, known as safety investigation board reports, aren't available to the public and cannot be used to punish a crew — they are in place to identify and quickly share any safety issues among the fleet.

To date, the staffers said they had received about 3,500 pages of documents, but information was redacted, leaving them unable to conduct oversight. The committee staffers spoke on the condition of anonymity to discuss sensitive matters.

The staffers said the documents they have reviewed left them concerned about whether Pentagon leadership has maintained a close watch on the Osprey program. Some of the problems with the aircraft date back a decade or more but still haven't been fixed.

After mechanical and material failures led to the 2022 Osprey crash in California, the military said it had instituted changes to prevent the issue from happening again.

"However, the recent fatal crash and ongoing investigations suggest that more transparency and rigorous testing is needed to verify these claims," Rep. Glenn Grothman, a Wisconsin Republican who chairs the committee, said in a statement to the AP ahead of the hearing Wednesday.

The Marine Corps is planning on using the Osprey through 2050, while Air Force Special Operations Command has already begun to talk publicly about finding another type of aircraft to conduct missions.

Osprey producers Bell Flight, the Boeing Co. and Rolls-Royce, which supplies the engines, are facing a new lawsuit from families of the five Marines killed in the 2022 California crash. The lawsuit alleges that the companies did not address known parts failures or safety issues that were a factor in the crash.

Boeing and Bell have declined to comment, citing the ongoing litigation.

The staffers said the Pentagon has not provided details on what the restrictions are as the aircraft returns to operations.

Besides the deadly crashes, there have been additional accidents in which the aircraft were destroyed and service members were injured, but none died.