The fatal crash of a US Air Force pilot during a training mission over Nevada has caused conspiracy theorists to speculate that the military was flying a mystery plane after officials refused to confirm what aircraft was in operation.

The crash, which killed Lt. Col Eric Schultz, occurred during a training flight on 5 September at the Nevada Test and Training Range around 100 miles northwest of Nellis Air Force Base, officials said.

However, what wasn't revealed was the model of aircraft being flown after it was stated the information as "classified and not releasable". The one piece of information the US military came from Air Force Chief of Staff David Goldfein who said: "I can definitely say it was not an F-35".

The statement ruled out one possibility but only sparked further intrigue.

One theory from skywatchers is that Schultz was flying the F-117A stealth fighter, which was taken out of service in 2008. There have been reports the US military has been flying a number of decommissioned jets in order to test out new stealth technology for future aircraft. Aviation enthusiasts photographed two F-117 Nighthawk jets in the air and on the ground at the Groom Lake test facility, otherwise known as Area 51, in July 2016.

However, according to Popular Mechanics, only Lockheed Martin pilots are allowed to fly the F-177A aircraft, which would eliminate this as a suspect.

The next prevailing theory is that the US military has obtained access to a Russian jet in order to test out its capabilities. There are several projects known to be in operation to understand and test the threat of opposing forces including the Russian Sukhoi SU-27, which uses many of the same engineering and technology of newer models in service.

Sukhoi 27 fighter jet
Could it have been the Russian Sukhoi Su-27 fighter jet? Wikipedia

Again, aviation fans have taken long range photos of the SU-27 above the Groom Lake area in simulated combat with US-built F-16s. It is speculated the US Air Force may even have had access to the newer SU-30, which it could have obtained in a trade with foreign forces such as India, Malaysia and Vietnam.

The US military has never admitted to owning Russian aircraft, which is why conspiracy theorists are quick to jump to conclusions as to why the aircraft in the crash is being kept so secret.

Talking of top secret, the wildest of claims coming from the foil-hatted community is that the US Air Force was testing a highly classified new stealth aircraft, known as a 'black' jet.

According to the Aviationist, the Northrup Grumman B-21 Raider Long Range Strike Bomber (LRSB) program, which would replace the current B-2 Spirit, is believed to be being tested in the Nevada area. In 2016 the Pentagon announced it had chosen a design to introduce the next-generation stealth plane yet there has been no official confirmation as to whether it even exists yet.

US Air Force B21 bomber
The US Air Force is looking for suggestions on what to call its latest stealth bomber. US Air Force

If it did, it would be more than likely testing would be carried out in the Nevada Test and Training Range, which is described as "the largest contiguous air and ground space available for military operations in the free world", with 2.9 million acres of land and 12,000 square miles of airspace to test equipment.

The incident follows the fatal crash of an F-16 fighter jet that occurred during a training mission on the same day in Arizona, killing an Iraqi pilot. Without further information from the US Air Force, the crash over Nevada remains a mystery.