Story highlights The oxygen system aboard one of the two VC-25 aircraft in Trump's Air Force One fleet was contaminated

Tools used to repair a plane's oxygen system must be cleaned to remove any residue

Washington (CNN) Three Boeing mechanics caused $4 million of damage and could have sparked a mid-flight fire while repairing one of the planes in the Air Force One fleet, a federal investigation has found.

The oxygen system aboard one of the two VC-25 aircraft available to President Donald Trump was contaminated while the plane was undergoing maintenance at a Boeing plant in San Antonio, Texas between April 1 and April 10, 2016, according to an incident report released by Air Force investigators this week.

The mechanics "caused the mishap by supplying and using non-oxygen clean tools, parts, components, a regulator, and an unauthorized cleaning procedure while performing oxygen system leak checks," according to the report.

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Tools and components used to repair a plane's oxygen system must be cleaned in a particular way to remove any residue that could react when it comes into contact with oxygen, the report noted.

If left unchecked, a contaminated oxygen system could increase the risk of fire or even cause an explosion.

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