The Air Force found that three Boeing mechanics at Boeing’s San Antonio, Texas depot contaminated the oxygen system on a presidential Air Force One VC-25A aircraft in April 2016. The investigation found the mechanics used contaminated tools, parts, and components and used an unauthorized cleaning procedure. The Air Force found Boeing “failed to exercise adequate oversight over the timeliness and quality of maintenance being performed” on the aircraft. The cost to remediate the contamination of the oxygen system was over $4 million, which was paid for by Boeing.