Microscopic examination supports early suspicions that wear and tear caused a fan blade to snap inside one engine of a United Airlines plane that made an emergency landing shortly after takeoff last month in Denver, federal safety investigators said Friday. The National Transportation Safety Board said the blade found “multiple fatigue fracture origins” on the inside surface of the hollow fan blade. The board said “multiple” secondary cracks were found, and that the examination is continuing. The Pratt & Whitney engine had been used on 2,979 flights since its last inspection, the NTSB said. After the Feb. 22 incident, the Federal Aviation Administration ordered that certain blades on the same model of engine on other planes be inspected for cracks before their next flight. The particular Pratt & Whitney engines involved are found on fewer than 100 Boeing 777 jets currently in use. They’re also on several dozen other planes …