Weeks after a UPS cargo plane crashed in Louisville, Ky., last November, a preliminary report by investigators identified the cause as a fractured bracket that led an engine to break off.

On Tuesday, the investigators revealed factors that appeared to have contributed to the part’s failure: infrequent maintenance requirements, siloed inspection information and ignorance among line mechanics and other key people about longstanding warnings that a bearing assembly within the bracket could fail.

Those details emerged in a National Transportation Safety Board hearing about the crash, which killed 15 people on the outskirts of the Louisville airport.

The bearing problem was known for years and was identified by Boeing, the manufacturer, in a series of service bulletins over a decade ago. The bulletins recommended periodic visual inspections and replacing the faulty apparatus with an updated mechanism avoiding the same fundamental weaknesses.

But those warnings may have been insufficient, investigators suggested, as they questioned representatives of UPS; the Federal Aviation Administration; Boeing; the Teamsters; and ST Engineering San Antonio Aerospace, which had conducted maintenance on the plane. Boeing never insisted that operators must replace the bearing assembly, and didn’t flag the problem as posing a potential flight-safety issue, some testified.

“Pre-accident, it was never scheduled to be replaced,” David Springer, the senior director of engineering at UPS, said of the problematic bearing, which, when it failed, ended up damaging lugs in the bracket assembly and set off a chain of catastrophic events. “It would have flown to failure.”

The Louisville crash harked back to the deadliest one in United States aviation history: the 1979 crash of American Airlines Flight 191 that killed all 271 people on board. That plane, a McDonnell-Douglas DC-10, was the forerunner to the MD-11 jet that crashed in Louisville, and it also went down when the left engine separated and fell to the runway.

The Louisville accident also raised questions about why, in later years, a string of problems with the MD-11’s bearing assembly hadn’t prompted more decisive action from the manufacturer or regulators — and why, in some cases, episodes appeared to have gone unreported to the Federal Aviation Administration.

Investigators zeroed in on four reports of failed bearings that were made to the F.A.A., in 2007, 2008, 2017 and 2020.

As investigators scrutinized those episodes, some — including the board chair, Jennifer Homendy — questioned why Boeing had never identified the problematic bearing as a “principal structural element,” a designation it had given the lugs holding the assembly together.

“There was a misunderstanding initially, 20 years ago, about the severity of the event that might result from the failure of this bearing,” said Melanie Violette, an F.A.A. engineer. “It was not believed to be critical to the integrity, the safety of the aircraft.”

Were it known that a bearing failure could severely damage the lug, “that would have changed the safety determination,” she added.

Henry Gallegos, the director of engineering at ST Engineering San Antonio Aerospace, said there were certain reasons to examine the bearings, which were encased in a sealant. But if the bearing “was just beginning to migrate, you would not be able to see it with that sealant installed,” he said.

As it was, the Louisville plane was up-to-date with its required maintenance schedule. But during the hearing, expert testimony revealed that the bearings could degrade even if no one was yet scheduled to look at them.

A maintenance technician told investigators that in 2020, while removing the engine on an MD-11 to reach an area of the wing he had to repair, he happened to see a bearing so corroded it had to be replaced.

But in transcribed interviews released by the N.T.S.B. on Tuesday, mechanics, quality control specialists and UPS’s own aircraft maintenance representative said they had been completely unaware of Boeing’s service letter regarding bearing maintenance. Their testimony strongly suggested they might not have known to look for degradation or damage outside of the specific visual check the F.A.A. requires every six years.

“I have never heard of that until now,” one mechanic who had been with ST Engineering for five years told N.T.S.B. investigators who asked about the published bulletin, according to transcripts. “That’s probably out of my job description,” said another, when asked if mechanics ever tried to look for potential bulletins or other airworthiness problems with the planes they serviced, outside of those flagged by UPS.

Two quality control officials for ST Engineering San Antonio Aerospace who were interviewed also professed ignorance about the existence of the service letter regarding the bearings. So did an aircraft maintenance representative for UPS. “I learned of that in Louisville a week and a half ago,” the representative said in December after the accident, according to an interview transcript. “I think somebody showed me that.”

In the aftermath of the Louisville crash, MD-11 planes were grounded and subjected to inspections. In the course of those checks, three were found to have cracks in the same bearing apparatus. But like the plane that crashed, they were not due for visual inspections.