A 33-year-old man has died and a 21-year-old man is missing after getting into difficulty while swimming near a waterfall in Snowdonia.
Another two, aged 27 and 25, were rushed to hospital after emergency services were called to the scene in Llanberis, North Wales Police said.
They received treatment and have since been released.
Police were alerted to the incident at 8.30am today and attended along with their water rescue unit, North Wales Fire and Rescue Service, paramedics and Llanberis Mountain Rescue Team.
RAF Valley confirmed that it had sent a helicopter to assist with the incident.
Superintendent Alex Goss said: "North Wales Police have been dealing with an incident near a waterfall in Llanberis after a multi-agency response was mobilised earlier this morning.
"The incident took place at a waterfall near the Snowdon Mountain Railway and involved four men who had been swimming in the area and had got into difficulty.
"I can confirm that one man aged 33 has died as a result of the incident.
"A second man aged 21 remains unaccounted for and a multi-agency search is currently taking place within the area."