President Donald Trump departing Air Force One at Andrews Air Force Base on January 26. AP Photo/Jose Luis Magana

Airline mechanics assigned to perform critical maintenance on the fleet of Air Force One planes used to fly President Donald Trump and previous presidents were caught using contaminated tools on the craft, causing more than $4 million in damage that could have caused a fire to break out on the plane, according to a federal investigation.

Three mechanics working for Boeing, which has a contract to perform critical maintenance on the Air Force One fleet, "failed to observe explicit warnings" while attempting to clean the plane at a Boeing-owned plant in San Antonio, Texas.

Boeing, which has caused a firestorm on Capitol Hill over its efforts to sell Iran billions of dollars' worth of planes that many suspect will be used to facilitate terrorism operations, caused more than $4 million in damage to the plane.

The dangerous cleaning methods employed by the Boeing employees could have caused a fire to break out on board the plane, according to an investigation, which refers to the three employees in question as "Mishap Mechanics," or "MM."

The shoddy work took place during routine maintenance checks performed from April 1 to 10 last year.

Three mechanics involved in the incident all were "assigned to the Presidential Airlift Squadron, Andrews Air Force Base, Maryland," according to the investigation. "The three Boeing mechanics were supporting the heavy maintenance contract between the United States Air Force and Boeing."

Investigators found that "three factors substantially contributed to the mishap."

"First, MM2 failed to observe explicit warnings concerning cleanliness while performing tasks on the [plane's] oxygen system," according to the investigation. "Second, Boeing failed to exercise adequate oversight over the timeliness and quality of maintenance being performed on the [aircraft]. Lastly, MM1, MM2, and MM3 failed to absorb or retain oxygen system training and failed to apply cleanliness procedures while performing oxygen system maintenance."

As of Thursday, "the cost to remediate the known contamination of the oxygen system is over $4 million, which was paid for by Boeing," according to the investigation. "There were no injuries as a result of the mishap."

A Boeing spokesman would not say whether the mechanics in question had been disciplined or fired, according to Defense One.

"We did complete some investigations alongside our Air Force partners," Davis told the publication. "Collectively, we did some corrective action reports so that we can ensure we have exemplary performance in the future."