A man died when he fell from a bridge over the M1 in Hertfordshire and a Good Samaritan driver who came to his aid was then killed by a passing vehicle.
Emergency services were called to the Bucknalls Lane bridge over the motorway near Watford at 10:45 GMT.
Police want to hear from anyone who saw the incident or who has information about the lead-up, a spokesman said.
Motorway CCTV footage showed emergency services on the bridge above the scene of the incident and on the road below.
The Good Samaritan was one of "a number of motorists" who had stopped to help, the spokesman added.
Both men died at the scene and the road was closed for six hours.
A silver Audi saloon was towed away from the scene. Police are yet to confirm the identities of the two dead men.
Both carriageways are now open.