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One of the president's Air Force One planes was put at risk when mechanics used contaminated tools during “heavy maintenance” last April, causing roughly $4 million in damages, according to the Air Force.

The contamination occurred in April of last year while President Barack Obama was still president. Because of unsafe cleaning procedures, there was a risk of fire breaking out on the plane.

Secret Service agents stand by as Air Force One departs from Dallas Love Field Airport March 12, 2016 in Dallas. Ashley Landis / Getty Images pool file

Three Boeing mechanics working on the plane, “failed to follow proper maintenance procedures leading to the contamination of the oxygen system,” according to a report by the Air Force Materiel Command. They were working on a VC-25A, one of two specially configured Boeing 747-200B aircraft, which are known as Air Force One when they fly the president.

Boeing repaid for the damages at its own expense.

After he was elected but before he assumed office, President Donald Trump threatened to cancel an order with Boeing for two updated 747’s that would join the Air Force One fleet.

"Boeing is building a brand new 747 Air Force One for future presidents, but costs are out of control, more than $4 billion,” he said on twitter. “Cancel order!"

But on his inaugural flight as president, he called one of the current versions “beautiful,” and a “great plane."