Federal investigators looking into last year's crash of a United Parcel Service Inc. cargo plane took an unusual step on Monday and expelled representatives of the company and the pilots union from the probe.

The National Transportation Safety Board said both the union and the company violated long-standing rules barring them from publicly analyzing causes of the August 2013 accident in Birmingham, Ala.

Two UPS pilots died when the jet slammed into a hill during a predawn approach to the runway. The investigation continues to spark debate over whether the cockpit crew's errors and failure to follow basic safety procedures were partly the result of fatigue. Information previously released by NTSB revealed the captain had complained to associates about feeling chronically tired and the pilots discussed work schedules during the flight.

The safety board hasn't yet determined the precise cause of the Birmingham accident. But the crash has become a flash point for those critical of the Federal Aviation Administration for excluding cargo pilots from more-stringent rules covering pilots flying for passenger airlines.

In a letter to the Independent Pilots Association, which represents UPS pilots, the NTSB said comments about fatigue-related issues improperly prejudged the results of the continuing probe and forthcoming federal findings.

The IPA declined to comment.

"We maintain that our actions have been in line with NTSB rules and guidance for communicating during an accident investigation," a UPS spokesman said. "We believe we have been unfairly reprimanded for attempting to set the facts straight and defending our brand."

Debate over the impact of fatigue on cargo operations has prompted the IPA and other labor groups to file suit and lobby U.S. Congress to tighten the requirements for cargo carriers.

UPS has consistently argued against such changes, saying the accident crew's schedule was consistent with pending revisions covering passenger airline crews. In a separate letter to the company, the NTSB's top lawyer said certain comments posted online by UPS violated rules of the investigation.

Debate over fatigue rules has even embroiled Jim Fraser, the government's Federal Air Surgeon, in controversy. At a safety conference in Washington D.C. earlier this month, Dr. Fraser, a senior FAA official, suggested he didn't see any medical or physiological reasons for leaving cargo pilots out of the latest regulatory changes.

"From a human perspective, there is no difference between" the two types of pilots, Dr. Fraser said, adding "there is a significant political side of the issue" because the cargo industry has complained the cost to comply with higher standards would be too high.

While potentially embarrassing for both sides, the move to exclude both company and union representatives isn't expected to have much impact on the probe's outcome because much of the technical work has been completed.

The decision is not without precedent. In 2011, NTSB officials removed American Airlines from participating in the federal probe of one of its jets that ran off the end of a runway in Wyoming.

Two years earlier, the safety board took similar action against the union representing air-traffic controllers involved in a fatal midair collision over the Hudson River between a sightseeing helicopter and a private plane.

Laura Stevens contributed to this article.

Write to Andy Pasztor at andy.pasztor@wsj.com

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