Two men are believed to have died after one jumped from a motorway bridge and another was hit after coming to his aid.
Hertfordshire Police and the East of England Ambulance Service were called to the "serious incident" on the northbound carriageway of the M1 near Junction 5 at about 10.45am.
It is understood two people died but officers are working to establish the circumstances surrounding the deaths.
Rupert Gadd, from the Hertfordshire, Bedfordshire and Cambridgeshire Police Road Crash Investigation Unit told Heart Radio: "A male is known to have jumped from the bridge parapet onto the carriageway.
"Other motorists have stopped to assist to his aid - unfortunately one of those people who stopped was struck by another vehicle.
"We have started an investigation into the collision and deaths of both males.
"We've handed the carriageway back to the Highways Agency - once they have done their final inspections it will re-open."
Footage taken from a motorway CCTV camera appeared on Twitter, which seemed to show emergency services working on a bridge above the scene of the incident as well as on the road below.
A police spokesman said: " Police are currently attending a serious incident and are asking drivers to avoid the area where possible.
"The southbound carriageway had been closed but is now open. However, the northbound carriageway is closed and is expected to remain closed for some time.
"Drivers are thanked for their patience but asked to find alternative routes where possible."
A spokesman for the ambulance service said two ambulances, an emergency response vehicle and an air ambulance had been dispatched to the scene but were stood down after midday.
In a statement issued at 3.45pm, a force spokesman said both carriageways had reopened.
Officers are appealing for witnesses to contact Hertfordshire Police on 101.