The search continues for a Japanese Air Self-Defense Force F-35A that went down off the coast of Honshu in northeast Japan last Tuesday. The pilot, Maj. Akinori Hosomi, remains missing.

Days after the Joint Strike fighter went missing on Japanese radar, bits of wreckage were found in the Pacific, confirming reports the aircraft crashed, but offering little evidence as to why. Now, a report published by the Japanese newspaper Mainichi Shimbun claims that Japan’s small fleet of F-35s experienced a number of issues—resulting in no fewer than seven emergency landings by five aircraft in recent months.

The country received a total of 13 F-35s thus far, with five experiencing in-flight emergencies that forced immediate landings. Despite reports painting this as an American failure, four of these five jets underwent final assembly not in American-based Lockheed Martin production facilities, but rather on Japanese soil by Mitsubishi Heavy Industries, Ltd. The F-35A that went down last Tuesday was also among the first jets assembled by Mitsubishi.

Currently, all of Japan’s F-35s are grounded.

In fact, last Tuesday’s crash wasn’t the first time that particular fighter ran into trouble. On two previous occasions, that F-35 experienced issues with both its cooling and navigation systems: once in June 2017, and again in August 2018. It remains unclear if these issues played a role in the aircraft’s disappearance last week.

Other issues that caused emergency landings included problems with the fuel and hydraulic systems. However, according to Japanese officials, the aircraft with these failures were diagnosed and received the necessary repairs to eliminate those issues.

Wreckage of missing F-35 found in the Pacific—pilot still missing

Read Next: Wreckage of missing F-35 found in the Pacific—pilot still missing

Prior to the crash, Major Hosomi accumulated more than 3,200 flight hours aboard military aircraft, with 60 of those hours in Japan’s new F-35s. The experienced aviator reportedly called off the training flight he and three other F-35 pilots were participating in just moments before the crash. However, the aircraft didn’t activate an automated distress signal it normally would following an ejection, suggesting Major Hosomi went down with the plane.

Although there’s great concern about the missing pilot, the majority of international attention is instead dedicated to the aircraft’s wreckage. The fighter represents only the second F-35 “total loss” to date, and the first time one belonging to a third-party nation has gone down. For Russia and China, two nations struggling to field their own fifth-generation fighters, finding parts of a downed F-35 is likely an opportunity too promising to ignore.

China’s J-20 stealth fighter, widely believed to be based on stolen plans of the United States’ F-22 Raptor, struggles with powerplant issues. Speculation is the Pratt & Whitney F135 engine that propelled the downed F-35A is of particular value. Russia’s stealth program appears to be lagging behind both the U.S. and China, which means Russian researchers could likely benefit from getting their hands on just about any component belonging to the advanced jet.

“The F-35A is an airplane that contains a significant amount of secrets that need to be protected,” Japan’s defense minister, Takeshi Iwaya, told reporters.