An 18-year-old has been arrested on suspicion of murder after a man died following a crash involving a car and an off-road bike.
Police were called to Pen Cae Crwn Road, Gorseinon, Swansea at 12:58 BST on Thursday where a man, believed to be an 18-year-old from Swansea, was pronounced dead at the scene.
Another man, aged 19, has been taken to hospital with injuries.
The man from Morriston has been arrested on suspicion of murder, driving a motor vehicle dangerously, driving a motor vehicle while unfit through drugs and driving a motor vehicle with a proportion of a specified controlled drug above the specified limit.
He remains in police custody.
The incident led to a major traffic build-up in and around Gorseinon, north-west of Swansea, which continued throughout Thursday afternoon.
A reporter on the scene said traffic was bumper to bumper, with several roads closed during rush hour traffic.
Residents were earlier told to stay in their homes.
The Welsh Ambulance Service said it was called to two incidents in Gorseinon on Thursday, one at 12:59 and another at 13:10.
It said a patient was taken to hospital in the first incident after an air ambulance was called to the scene.
Det Ch Insp Danielle Thorne, from South Wales Police, thanked the local community for their patience and understanding.
“There is significant police activity in the area, and will continue to be for several hours, however we are working at pace to conduct enquiries in this complex investigation,” she said.
She added that Pen Cae Crwn Road is currently closed between Cecil Road from the junction of High Street to Pen-Cae Crwn Road to the junction of Frampton Road, while Brighton Road, Brynteg Road and Pontardulais Road are also closed.
“We have significant resources working to lift these road closures as soon as possible,” she added, urging those in urgent need of entry to speak to officers at the scene.
She also urged witnesses to get in touch with police.