Three mechanics failed to follow proper cleaning procedures on a VC-25A undergoing regular heavy maintenance, causing contamination of the aircraft’s oxygen system, according to an abbreviated accident investigation board report released late Monday. The contamination occurred at Boeing’s Port San Antonio facility in Texas in April 2016 and was discovered after an unapproved regulator was found connected to the passenger oxygen system, according to the report. Boeing covered the $4 million cost to repair the aircraft, known as Air Force One when the President is on board. No injuries were caused by the mishap. The aircraft, one of two specially configured Boeing 747-200B aircraft, is assigned to the 89th Airlift Wing at JB Andrews, Md.