Charleston, W.Va. (AP) -- The National Transportation Safety Board is examining the oxygen system on a single-engine plane that crashed into a remote West Virginia hillside in July.

Pilot William Huff was killed, but it wasn't clear if he died before the crash or as a result of it.

In a preliminary report, the NTSB says the 61-year-old Indiana man was incoherent and breathing heavily just before controllers lost contact with him near Indianapolis.

The plane had left York, Nebraska, and was expected to land at Eagle Creek Airpark near Indianapolis but the plane flew on for more than 300 miles before crashing near Ravenswood, 50 miles north of Charleston.

Huff was the only person on the plane.

The NTSB said Huff was an experienced pilot who passed an air worthiness physical just days before the July 30 crash.

A final report isn't expected for several months.