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.
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.
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.
The incident had previously been reported by the Air Force Times
The investigation was triggered when one of the Boeing mechanics notified his superiors that something looked awry, according to Boeing.
"We took swift action to self-report the incident to the US Air Force. The oxygen system was remediated by Boeing at no cost to the government," the company's statement said.
The US Air Force has completed a number of independent inspections on the aircraft and determined that it meets all FAA Federal Aviation Administration and US Air Force requirements, according to Boeing, which added that the plane has returned to the Air Force and is preparing for presidential service.
Boeing has a contract with the US Air Force to not only build the President's aircraft but also to perform critical maintenance on both planes in the fleet. The damaged aircraft is operated by the Presidential Airlift Squadron at Andrews Air Force Base in Maryland.
But the potentially dangerous outcome of failing to use proper cleaning procedures on the President's aircraft has raised questions about Boeing's training and oversight methods. Investigators found that the mechanics "failed to observe explicit warnings concerning cleanliness" while working on the oxygen system -- one of three major factors identified as leading to the contamination.
The incident report also noted that Boeing failed to exercise adequate oversight over the repair procedures, and the mechanics failed to learn the information taught during oxygen system training.
While Boeing would not comment on whether the mechanics mentioned in the report remain employed by the company, spokesman Ben Davis did tell CNN that they have "completed multiple investigations and corrective action plans so that we ensure stellar performance in the future."
The two Air Force One jets
currently in use were ordered by President Ronald Reagan -- the interiors were selected by first lady Nancy Reagan -- and were delivered in 1990 during George H.W. Bush's administration. Both are getting increasingly expensive to fly and maintain.
Boeing is currently developing a replacement Air Force One model -- a project that famously caught the attention of President-elect Trump late last year.
"Boeing is building a brand new 747 Air Force One for future presidents, but costs are out of control, more than $4 billion. Cancel order!" Trump tweeted at the time.
Within hours of the tweet, Boeing CEO Dennis Muilenburg
was on the phone with Trump for the first time, congratulating the President on his election win, committing to control the jet's cost and extending an invitation to Trump to visit his company's factories, according to one person familiar with the call.
Boeing has said it currently has an Air Force One contract worth $170 million.