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At least nine people have died and 11 others were hurt following a freight aircraft crashed during departure from an international airport in Kentucky's largest city on Tuesday evening.

Aircraft specialists believe the plane crashed after one engine failed and another appeared to be damaged while lifting off.

The exact cause remains unknown what caused the plane to crash, leading to a massive fireball to ignite after it couldn't become airborne from the airstrip. Footage showed fire had already engulfed a single wing of the aircraft while it was attempting to take off, potentially spreading through the plane and caused the explosion, or the plane might have ignited after colliding with an object on the ground.

What's clear is that the 144,000 litres of jet fuel on board the MD-11 jet needed for the flight probably intensified the blaze, which rapidly extended to multiple structures beyond the runway and continued burning for hours.

How did it start?

UPS uses the Louisville air hub as a logistics center for its global operations and its Flight 2976 was at the start of a 4,300 mile journey to Honolulu in Hawaii when the freight aircraft attempted to take off.

Tracking information from flight monitoring services shows the plane began to taxi along the designated runway at around 5:15 PM local and managed to reach a maximum velocity of 344km/h.

However, confirmed videos demonstrate that by the time the plane achieved this velocity a fire had completely engulfed its left wing and the plane couldn't climb before the massive explosion.

Authorities issued a shelter-in-place order to area inhabitants and scrambled numerous fire crews to the location.

The state's governor confirmed details seen in CCTV footage that shows the aircraft flying just metres off the ground before a bright flash consumed the aircraft. It is then seen slamming into the terrain as a massive fire explosion erupts around it about a minute into its journey.

A verified clip taken by drivers on adjacent roads showed the flames erupting into the skyline while later videos showed smoke pouring from the area.

Overhead photographs shown on local media showed wreckage scattering across the airstrip and falling onto the tops of at least two local businesses.

Potential causes of the accident

Air traffic control communications reviewed are mostly unclear and filled with static so no clear dialogue can be heard about the crash as it unfolded.

However, experts suggested that a catastrophic malfunction of two of the engines may have been responsible for the disaster.

The trijet cargo aircraft employs three powerplants. Two are installed under the airfoils, and a third is positioned in the tail at the base of the vertical stabilizer.

Footage confirmed showed a blaze engulfing the left wing of the plane, which then leaned leftward as it attempted to gain lift and become airborne.

Multiple specialists independently suggested the left engine may have detached from the plane after suffering from a mechanical or structural failure.

Additional photographs taken after the crash showed a charred engine resting on the grass next to the runway.

An experienced former aviator and aircraft safety specialist said that it was "extremely rare" for an powerplant to separate in flight.

The expert recalled the 1979 aviation disaster tragedy, in which 273 individuals were killed when the aircraft's powerplant separated as it took off from O'Hare International Airport. Parts of the engine had been damaged when it was replaced on the plane, but the expert stated it was too early to say whether a similar fault led to the powerplant separation on the MD-11.

The expert said the cargo plane would have been able to fly with just two engines but the damage resulting from the fire on the left wing was likely so great it resulted in the plane's third engine to lose thrust.

"Given the severe nature of the incident we cannot determine what other damage was done when the engine came adrift," the expert commented.

A different aircraft specialist observed the video evidence seemed to indicate the remaining powerplant had been damaged because it released smoke. The issue could have occurred while it was struck by wreckage from the blaze and the powerplant separation.

"The tail-mounted powerplant that expelled a puff of smoke appears to wind down almost immediately afterwards," the expert stated. "This left just the starboard powerplant generating power, creating a significant thrust asymmetry and leaving the aircraft unable to gain height.

"Failing two powerplants during take-off leaves the aircraft with only a fraction of its thrust and little chance of maintaining flight, particularly under maximum take-off weight," the specialist continued.

Why did the crash cause such damage?

Post-accident videos revealed a situation of total disorder with numerous blazes burning across a extensive area of the site and smoke billowing into the sky.

The aircraft, which was 34 years old and had been operated for passenger travel until 2006, had already completed one return journey from Louisville to Baltimore in Maryland on Tuesday.

It has not been confirmed what cargo was aboard the Hawaii-bound aircraft, though officials stated the plane was not carrying anything that would create a heightened risk of environmental hazard.

"This involved extended freight transport from Louisville to Honolulu, so the trijet was transporting a significant fuel quantity," an expert said. "The substantial fuel quantity not only reduced performance but also accounts for the massive explosion seen after the crash."

Officials informed media that the plane was carrying 144,000 litres of fuel for the long journey when it crashed. The fire was likely amplified on the surface because the aircraft slammed into a fuel processing facility adjacent to the airfield.

A specialist indicated authorities will concentrate on how the initial fire began, and "whether debris struck the central powerplant, and if previous repairs on the left engine were a factor." He added: "Atmospheric conditions were favorable, so environmental factors are unlikely."

The federal investigation agency has sent a team to the site and will take charge of the probe into the accident's origins, though such inquiries may require years to finalize.