Federal investigators have found the pilot of a crop- duster plane that crashed in San Joaquin County. But the National Transportation Safety Board on Friday offered no explanation about why neither he nor the company that owns the plane reported the incident as required by federal regulations. The single-engine plane was spotted upside-down and broken apart in marshland near the town of Byron by a pilot flying over the area Feb. 25. Authorities say the plane had crashed two days earlier and was empty with some blood inside when they arrived. A witness told investigators that the unidentified pilot had walked away and was picked up at the scene.