BISMARCK, N.D. (AP) — An air ambulance that crashed in western North Dakota this month and killed all three people on board didn’t send out a distress call before it apparently broke apart in midair.

Morton County Sheriff Kyle Kirchmeier says what caused the breakup is still a mystery, but there’s no indication the Cessna 441 turboprop airplane was struck by anything and there was no fire on board. Weather also is not believed to have been a factor. The National Transportation Safety Board is investigating the cause.