Two people have died following an incident at Bournemouth beach on Wednesday afternoon (31 May).
Dorset Police said a 17-year-old boy and a 12-year-old girl sustained critical injuries and died in hospital.
A man aged in his 40s, who was on the water at the time, has been arrested on suspicion of manslaughter.
Eight others were treated by the ambulance service for injuries that are not life-threatening during the incident, which saw emergency services descend on the beach shortly after 4:30pm.
An investigation into the circumstances surrounding the incident has been launched with Dorset Police working alongside the Marine Accident Investigation Branch and the Maritime and Coastguard Agency in accordance with established protocols.
Detective Chief Superintendent Neil Corrigan, of Dorset Police, said: “Our thoughts are with the loved ones of the young people who tragically died and we are doing all we can to support their families.
“I understand the beach was very busy at the time of the incident and I would ask anyone with information that may assist our enquiries to please come forward.
“We are at the early stages of our investigation and would ask people not to speculate about the circumstances surrounding the incident.”
Video shared with ITV Meridian shows a large emergency response at the beach. Credit: Col's Place.
In a statement issued directly to ITV News Meridian earlier, the Coastguard said: "HM Coastguard has been assisting South West Ambulance Service after a call was received from Poole Bay RNLI Lifeguards about people in difficulty in the water.
"Coastguard helicopter from Lee-on-Solent, two air ambulances, Poole and Southbourne coastguard rescue teams, Mudeford RNLI lifeboat, Dorset Police were all sent in support.
"Two people had been pulled from the water and passed into the care of the ambulance service.
"Coastguards searched to make sure there were no other people missing and are satisfied there are not."
An eyewitness reported seeing a group of teenagers jumping from the pier into the water. He said it appeared that they were being pulled under the pier by the strong current.
He added that two people were seen being helped to shore by members of the public, and treated by medical crews.
The leader of Bournemouth, Christchurch and Poole Council, Cllr Vikki Slade said: “We are heartbroken to hear the news that two young people rescued at Bournemouth Beach earlier today have died, and our thoughts are with their family and all those involved in the rescue.”
A spokesperson for the South Western Ambulance NHS Foundation Trust (SWASFT) said: “We were called at 16:07 on Wednesday 31 May to a sea incident in Bournemouth.
"We sent x2 air ambulances, x6 double-crewed land ambulances, x1 critical care car, x2 operations officers, x1 doctor, x1 hazardous area response team and x1 responding officer.
“We conveyed two patients to Royal Bournemouth Hospital and Poole Hospital.”
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