An elderly woman has died and a number of other people have suffered injuries as a result of a motorway collision between Moira and Lurgan on Friday.
The M1 will be closed through the night while investigators examine the scene.
Many motorists were left stranded for almost five hours in the long tailbacks with some taking to the hard shoulder in an effort to escape the gridlock.
One driver told the News Letter he was stationary from 5.30pm until 10.10pm when the first police officer approached with information or instructions.
The motorist was eventually advised to carefully turn around and drive back towards Moira.
Police have said they understand the prolonged closure has caused inconvenience, but said it is necessary to allow for a thorough investigation.
Anyone who witnessed the collision is asked to contact the PSNI on the non emergency 101 number.