HRO theory flowed out of Normal Accident Theory, which led a group of researchers at the University of California, Berkeley (Todd LaPorte, Gene Rochlin, and Karlene Roberts) to study how organizations working with complex and hazardous systems operated error free. They researched three organizations: United States nuclear aircraft carriers (in partnership with Rear Admiral (ret.) Tom Mercer on the USS Carl Vinson), the Federal Aviation Administration's Air Traffic Control system (and commercial aviation more generally), and nuclear power operations (Pacific Gas and Electric's Diablo Canyon reactor).