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, CNN reported, citing a federal investigation.

The oxygen system on one of the two VC-25 aircraft was contaminated while the plane was being repaired 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.

Last year's mishap did not result in any injuries and the aerospace contractor paid for the damages.

"Boeing fully understands the level of responsibility that comes from working on the President's aircraft," Boeing told CNN in a written statement.

Air Force One .

Credit: MANDEL NGAN

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Credit: MANDEL NGAN