Police in Scotland have launched an investigation after a couple was found in a crashed car three days after reports of a crash near a busy highway.

Authorities say that a dead man and seriously injured woman were found off the roadway in the car on Wednesday near the city of Stirling. An investigation is underway into the failure to follow up reports of a crash on Sunday morning.

Assistant Chief Constable Kate Thomson says in a statement that "for reasons currently being investigated, that report was not followed up at the time."

The injured woman is in critical condition in a hospital in Glasgow.

The accident is believed connected to an ongoing missing persons investigation involving 28-year-old John Yuill and 25-year-old Lamara Bell, who were last seen on Sunday.