Malaysia Airlines Flight 370 isn’t the only large aircraft to vanish mysteriously without a trace. Four U.S. Air Force Predator drones have disappeared into thin air while flying over Afghanistan – never to be seen again.

The still-missing, but presumed dead, aerial robots were among 400-plus large U.S. military drones involved in major accidents around the world between 2001 and 2013, according to a yearlong Washington Post investigation.

Each of the four Predators was armed with Hellfire missiles when they took off into the wild blue yonder, according to Air Force accident-investigation records obtained by The Post.

In each case, the pilots who were flying the Predators by remote control from the ground lost their satellite links with the aircraft, so they couldn’t communicate. The drones’ transponders also stopped working and they didn’t show up anywhere on radar. That can occur if there’s a sudden and complete loss in electrical power – caused by, say, a giant bolt of lightning. But without any eyewitnesses or physical evidence or witnesses, Air Force investigators could only guess at what happened in these instances:

Date of disappearance: July 21, 2008
Identity: MQ-1B Predator, tail number 05-3135
Last known location: About 100 nautical miles north-northeast of Kandahar Air Base
Estimated loss: $3.8 million
Unit: 3rd Special Operations Squadron, 27th Special Operations Wing, Air Force Special Operations Command

An armed Predator presumably crashed after it lost link and disappeared while flying over Afghanistan during a power loss to the ground-control station at Cannon Air Force Base in New Mexico. Two other Predators flying in Afghanistan lost their links at the same time because of the power failure. The crew regained power and was able to regain links to those two drones. The Predator that disappeared was pre-programmed to return to base in the event of a lost link but for some reason did not. Investigators speculated that it flew into hazardous weather and crashed. The aircraft should have returned to base automatically after the lost link, but it disappeared from radar and ground crews never found it

Date of disappearance: May 13, 2009
Identity: MQ-1B Predator, tail number 07-3183
Last known location: somewhere over Afghanistan, exact location classified
Estimated loss: $3.9 million
Unit: 15th Reconnaissance Squadron, 432nd Wing, Air Combat Command

An armed Predator mysteriously vanished in Afghanistan about five hours into a flight. The skies were clear and the aircraft was flying well above a mountain range when the return satellite data link was lost to Creech Air Force Base in Nevada. The transponder signal was lost three minutes later. Investigators concluded that there was no evidence of bad weather, icing, fuel loss or pilot error. A sergeant raised the possibility that the aircraft could have had its satellite links hacked or hijacked, saying: “If somebody else was on the same frequencies, they could have taken over the aircraft or just knocked it out completely.” Investigators found no evidence of hacking.

Date of disappearance: June 5, 2011
Identity: MQ-1B Predator, tail number 07-3204
Last known location: 60 nautical miles northeast of Jalalabad; very close to Pakistani border
Estimated loss: $4.4 million
Unit: 20th Reconnaissance Squadron, 432nd Air Expeditionary Wing, Air Combat Command

An armed Predator was apparently struck by lightning and presumably crashed. The aircraft lost satellite links after the lightning strike and was not recovered. The crew knew it was flying in the vicinity of thunderstorms but had an urgent mission to assist ground troops in the area who were under enemy fire.

Date of disappearance: July 10, 2011
Identity: MQ-1B Predator, tail number 06-3174
Last known location: about 20 miles south of Jalalabad
Estimated loss: $4.4 million
Unit: 3rd Special Operations Squadron, 27th Special Operations Wing, Air Force Special Operations Command

An armed Predator disappeared and presumably crashed. It was the third Predator crash near Jalalabad in five weeks. In each case, the drones lost their satellite links and did not fly their emergency pre-programmed mission. Unlike the other accidents in the vicinity of Jalalabad, the weather in this case was clear and investigators were unable to determine a cause. They found no evidence of maintenance or operational problems prior to the lost link. The aircraft had been flying for about 19 hours but still had 138 gallons of fuel in the tank.