Police have said that a man was reported making off from the scene of a crash in the Newry area, while a driver remained trapped inside.
The PSNI said the two-vehicle collision happened on the Burren Road, just outside Warrenpoint, on Friday at around 2.20pm.
Inspector Stephen Graham said in a statement that the collision left an 18-year-old man in a critical condition in hospital.
It involved a grey BMW car and a white van.
He said: “A police patrol car travelling on the Burren Road came upon the scene of the collision.
“One passenger from the BMW is reported to have been seen making off from the scene across nearby fields whilst the driver of the car remained trapped in the vehicle.
“Due to the serious nature of his injuries he was transferred to hospital in Belfast by the police helicopter.
“The driver of the van was treated for non-life threatening injuries at a local hospital.
“The Burren Road was re-opened to traffic at around 10pm. The office of the Police Ombudsman was informed of this incident.”
He added: “I would ask the passenger from the BMW who made off from the scene, or anyone with any information about the collision, to contact police in Newry on 101.
“Information can also be given anonymously through the independent charity Crimestoppers on 0800 555 111.”