Airline mechanics caused $4 million in damage to an Air Force One plane in 2016, according to a Department of Defense investigation released Wednesday.

Three Boeing Co. mechanics “failed to observe explicit warnings” and contaminated one VC-25A plane’s oxygen system, increasing the risk of fire, according to the report.

The VC-25A model, built especially to transport presidents, requires “oxygen clean” equipment and regulators to prevent risk of fire.

Mechanics performing heavy maintenance on the plane in San Antonio, Texas “supplied and used contaminated tools, parts, components, a regulator, and an unauthorized cleaning procedure while performing oxygen system leak check,” the report says.

The first mechanic supplied contaminated parts for the oxygen system, which the second mechanic worked on installing. A third mechanic tried to help by providing “cleaning solution in an unauthorized procedure in an attempt to sanitize these parts.” The second mechanic installed the contaminated parts, which affected the whole system.

The Air Force accident investigators said “the cost to remediate the known contamination of the oxygen system is over $4 million, which was paid for by Boeing.”

“We took swift action to self-report the incident to the Air Force, and we remediated the oxygen system at no cost to the government,” Boeing spokesman Ben Davis told the Air Force Times.

“We fully understand the level of responsibility that comes from working on the president’s aircraft, and we’re committed to our partnership with the Air Force to provide the highest standard of support for the VC-25.”

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