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Washington • The Air Force is blaming Boeing mechanics for accidental contamination of the oxygen system aboard one of the modified Boeing 747 planes known as Air Force One used for presidential travel.

The plane was undergoing maintenance at a Boeing facility in San Antonio, Texas, when the mishap occurred in April 2016. No one was injured.

An Air Force accident investigation report released this month faulted three Boeing mechanics for using tools and parts that did not meet cleanliness standards while checking oxygen lines for leaks. The standards are meant to prevent a fire hazard. The mechanics also used an unauthorized cleaning procedure.

The report says Boeing has paid more than $4 million to clean or replace contaminated portions of the aircraft's oxygen system.