The phrase "mysterious circumstances" refers to a situation where something happened or is happening without a clear or easily explainable reason. It suggests that the details surrounding an event are puzzling or unknown. Full definition
After her husband dies under mysterious circumstances, a woman takes over his church to defend her children and religious beliefs. (rottentomatoes.com)
He died in mysterious circumstances under the responsibility of the British state. (politics.co.uk)
The town's founder recently died under mysterious circumstances. (gonintendo.com)