Records show investigators didn't perform tests for cyanide or carbon monoxide poisoning. National Transportation Safety Board inspectors said it was impossible to perform those tests with the amount of tissue available.
National Transportation Safety Board investigator Jason Aguilera said officials have found no evidence Branstetter was affected by carbon monoxide poisoning, which can occur in some airplanes when exhaust fumes get into the cockpit from the plane's cabin heat system.
However, investigators found no problems with the plane's cabin heat system, Aguilera said, making it unlikely that exhaust fumes could have entered the cabin.
NTSB inspectors have said they have ruled out weather as a factor and have also said there's no indication the plane ran out of fuel before the crash. Inspectors have said it could be up to a year before a final NTSB report, including a cause of the crash, is available.
Branstetter, a former state senator, was an accomplished pilot and a licensed flight instructor.