By Andy Pasztor 

Federal accident investigators looking into the crash of a United Parcel Service Inc. cargo plane a year ago took the unusual step of expelling both company and pilots union representatives from the probe.

In Monday's announcement regarding the August 2013 accident in Birmingham, Ala., the National Transportation Safety Board said the pilots union and the company violated long-standing rules barring them from publicly analyzing causes of the crash.

Both 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. According to information previously released by NTSB, the captain had complained to associates about feeling chronically tired.

The safety board hasn't yet determined the precise cause of the Birmingham accident. But already, it 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 carriers.

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

The IPA declined to comment.

A UPS spokesman said, "We maintain that our actions have been in line with NTSB rules and guidance for communicating during an accident investigation...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 Congress to include cargo carriers in the current rules.

UPS, on the other hand, has consistently argued against such changes. In a separate letter to the company, the safety board's top lawyer said that certain comments posted online by UPS violated rules of the investigation.

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

In 2011, NTSB officials also removed American Airlines from participating in the federal probe of one of its jets that ran off the end of a Wyoming runway.

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

Laura Stevens contributed to this article.

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

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