A Japanese F-35 stealth fighter jet with one pilot on board disappeared from radar while on a training mission over the Pacific on Tuesday night, the defense ministry said.

The fighter jet went missing at around 7:30 p.m. (1030 GMT) as it was flying some 135 km (84 miles) east of Misawa in Aomori Prefecture in northeastern Japan, a ministry spokeswoman said.

It was not immediately clear if it had crashed, the spokesperson said, adding: “We are still trying to search for the aircraft.”

Update, April 10: Wreckage from the jet’s tail was found and recovered from the sea late on Tuesday night, and the Japan Air Self-Defense Force concluded that the F-35 had crashed.

Defense Minister Takeshi Iwaya told reporters on Wednesday morning that the pilot had signaled that he needed to abort the training mission shortly before communication was lost and the aircraft disappeared from radar.

The plane lost contact about 30 minutes after taking off from Misawa Air Base, public broadcaster NHK reported.

The Air Self-Defense Force said the F-35A was flying with three other aircraft, at the time, Kyodo News reported.

“The cause is unknown,” Defense Minister Takeshi Iwaya said, adding that the remaining 12 F-35As deployed to Misawa were grounded.

The fate of the pilot was not immediately clear.

The Self-Defense Force and coastguard dispatched vessels to carry out rescue operations, NHK added.

On Wednesday morning, the Self Defense Force said that parts of the missing F-35 jet’s tail had been found in the sea, public broadcaster NHK reported

Japan’s first F-35A fighter squadron based at Misawa became operational on March 29. It plans to deploy a total of 105 F-35A conventional takeoff and landing jets and 42 of F-35B short take-off and vertical-landing variant.

With reporting from AFP. This post was updated on April 9.