The crash occurred about 1:30 p.m. Wednesday.
In a news release Thursday, the Custer County Sheriff's Office identified the passenger as 48-year-old John Webb. The pilot, 33-year-old Dustin Webb, who was flown to a hospital for treatment.
It's not clear whether or how the two were related. They're both from Huntsville, Texas. A sheriff's office spokeswoman didn't immediately return messages from The Associated Press.
The National Transportation Safety Board is leading the crash investigation. An NTSB representative didn't immediately return an Associated Press call.
The sheriff's office says the two-engine aircraft was headed from North Dakota to Texas when the crash occurred.