On Tuesday, at approximately 7:30 p.m. local time, a Japanese Air Self Defense F-35A Joint Strike Fighter disappeared from Japanese tracking systems somewhere off the coast of northern Japan. According to local media reports, a search and rescue mission is underway. While not confirmed to be a part of this effort yet, a number of Japanese vessels have moved into the area, likely in search of the downed fighter.

The aircraft was one of four Japanese-flagged F-35s conducting training operations roughly 83 miles east of Misawa Air Base at the time of its disappearance.

Japan’s fleet of F-35s promises to become the largest outside of the U.S., with 147 of the aircraft already on order. Japan’s first operational F-35 squadron was just stood up out of Misawa Air Base on March 26th, comprised of 13 total fighters.

The few limited statements issued thus far by Japanese officials all seem to suggest the aircraft went down. The Japan Times reports that airplanes and helicopters are already joining warships in the vicinity—likely those three vessels identified by ship traffic software above—to continue the search and rescue. When Japan’s Defense Minister Takeshi Iwaya was asked about the cause of the incident, he responded only that every effort must be made to save the pilot’s life first before speculation into causes can begin. Thus far, there has been no word about whether the pilot ejected from the plane.

Despite troubling reports in recent months about the F-35s operational readiness, only one other F-35 has been lost thus far. Last September, an F-35B (vertical landing variant) crashed near Marine Corps Air Station Beaufort in South Carolina. The pilot was able to safely eject from that crash.

Although the internet is ripe with rumors about a possible defection of the Japanese pilot to nearby Russian airspace, the behavior of the Japanese government since the disappearance doesn’t seem to support that theory. With three other F-35s in the region when the aircraft disappeared, those pilots likely would have seen what happened to the fourth aircraft in their formation and reported the incident to command. Had the lost pilot attempted to defect, it’s unlikely there’d be a search and rescue mission mounted in the aircraft’s last known area.

Japan’s Air Self Defense Force has grounded its remaining 12 F-35s while the investigation continues.