This picture taken on June 5, 2017, shows the first F-35A stealth fighter assembled in Japan, unveiled at a Mitsubishi Heavy Industries factory in Toyoyama.
Tokyo CNN  — 

Wreckage from a Japanese F-35 stealth fighter that crashed into the Pacific almost a month ago has been recovered, Japan’s defense minister said Tuesday, but the reason for the loss of one of the world’s most advanced warplanes remains a mystery.

Pieces of the jet’s flight recorder and cockpit canopy have been lifted from the ocean floor, Defense Minister Takeshi Iwaya told reporters in Tokyo.

He described the condition of the flight recorder as “terrible” and said the device’s memory part was missing, leaving investigators still wondering how the US-designed jet crashed April 9.

The jet and its pilot, Maj. Akinori Hosomi, went missing while on a training mission from Misawa Air Base in northern Japan.

Hosomi, a 41-year-old with 3,200 hours of flight experience, was last heard from when he signaled to his squadron mates that he would have to abort the mission before his craft disappeared from radar.

A Japan Coast Guard's vessel and US military plane search for a Japanese fighter jet, in the waters off Aomori, northern Japan, Wednesday, April 10, 2019.

Pieces of the plane’s tail fins were recovered from the sea shortly thereafter, but noting else turned up until May 3, when the US Navy-chartered deep-sea diving support vessel Van Gogh collected a part of the flight recorder, Iwaya said Tuesday.

The US-chartered ship was acting on information gleaned by Japan’s deep sea research ship Kaimei, which carries a remotely operated submarine and equipment to grab samples from the seabed.

Japan’s Defense Ministry did not reveal exactly where or at what depth the latest pieces were found. The plane was east of Misawa Air Base over the Pacific when it disappeared.

The Defense Ministry said the search for more wreckage would continue and includes a Japanese destroyer as well as chartered commercial salvage ships.

A Japanese F-35A arrives at Misawa Air Base in northern Japan in 2018.

Japan grounded the other 12 active F-35s in its fleet after the crash. They remain out of action while the investigation continues, the Defense Ministry said.

With 147 of the $100-million-plus F-35s on order, Japan intends the planes to be the mainstay of its air forces for decades to come, and officials have said since the crash that their faith in the program has not wavered.

The jet that crashed was the first off a Japanese assembly line for the F-35A, one of three variants of the plane. Japan had announced plans before the crash to stop production there in 2022 and import the rest of its fleet from the US.

The United States has hundreds of the jets in its fleets and on order, as do a dozen other countries.

Those jets continue in operation. US Air Force F-35As, the same type as the Japanese jet that crashed, have since flown combat missions in the Middle East.