(Reuters) -The National Transportation Safety Board said on Thursday it has found evidence of fatigue cracks in a key part of a UPS MD-11 cargo jet that crashed this month in Kentucky, an important clue into the cause of the incident that killed 14.  

Air safety experts said the discovery of fatigue cracks in a support structure on the left pylon, which connects to the wing and the plane's engine, is a significant finding in the investigation into the November 4 crash in Louisville.

Besides evidence of fatigue cracks, which preceded the accident in the left pylon aft mount lug, the NTSB also said in its preliminary report that it found areas of overstress failure.

As a result, one side of the pylon's supports failed due to fatigue, and when it broke, the other side could not handle the extra load.

"That's a major clue," said U.S. air safety expert Anthony Brickhouse of the cracks, which he added developed over multiple flights.

CRACKS WEAKEN SUPPORT STRUCTURE: EXPERT

U.S. safety expert John Cox said such fatigue cracks weaken the support structure.

"It finally gets to a point where the force overcomes what the structure can withstand and that's a point of failure," Cox said. "There is a significant likelihood that that's what occurred here."

Brickhouse said investigators need to figure out why the crack was not discovered as part of the plane's maintenance, a search that could go back for years.

U.S. safety investigators have already said they are probing the maintenance history of the 34-year-old UPS cargo plane that was in Texas for repairs weeks before crashing in flames after takeoff. That maintenance was done at a site belonging to Singapore's ST Engineering.

According to the report, the plane had not flown enough cycles to warrant special detailed inspections of the part.

The preliminary report referenced the crash of an American Airlines DC-10 jet in May 1979, which it listed under "similar events." During the takeoff rotation of Flight 191 from Chicago O'Hare Airport, the left engine and pylon assembly, and about three feet of the leading edge of the left wing, separated from the airplane and fell to the runway.

The NTSB found evidence of fatigue cracking in that incident.

UPS and FedEx grounded their MD-11 fleets this month as a precaution and at the recommendation of U.S. planemaker Boeing. The Federal Aviation Administration also issued directives that temporarily grounded the MD-11 and the DC-10 due to their similar design.

NTSB Chair Jennifer Homendy told Reuters this week that Boeing is doing some additional modeling and testing as part of the investigation. Boeing has owned the broader MD-11 program since its 1997 merger with McDonnell Douglas.

Preliminary reports are normally published around 30 days after an incident, with a more comprehensive final report unveiled about a year after the crash.