The National Transportation Safety Board is investigating the source of water found in the engine and part of the fuel system of a Piper Cherokee Six that crashed near Jackson, Miss., last month, killing three Civil Air Patrol members. The plane had not been flown in two months before the crash, but the pilot conducted an unusually quick preflight inspection and apparently didn’t notice that one tire was low on air, according to witnesses. The pilot and two passengers, all experienced pilots and Civil Air Patrol officers, were running late to an FAA safety meeting about 30 miles away. Witnesses said the plane did a quick run-up but that otherwise the takeoff appeared normal. At about 1,000 feet, while talking to air traffic controllers, the pilot said the plane had an engine problem and that it was returning to the airport. But the plane crashed into a house and was extensively damaged by fire, killing all three men on board. In testing the plane’s engine, the NTSB said it found dirty water in the cylinder valves, likely from firefighting efforts. But the fuel divider had clear water in it, and the NTSB is still trying to determine where that water came from.