Two children killed in the sea off Bournemouth may have been overcome after being hit by the wash from a jet ski or boat.
Police investigating the deaths of the 12-year-old girl and 17-year-old have revealed they are working on the belief that there was no actual contact between "a vessel and any swimmers" at the time of tragedy shortly after 4.30pm on Wednesday.
They were pulled from the water with "critical injuries" and died in hospital.
Dorset Police said: "Following enquiries, a man aged in his 40s who was on the water at the time has been arrested on suspicion of manslaughter."
Initial beliefs that the incident was connected to teenagers tombstoning - or jumping - from the pier is believed to have been ruled out. Witnesses said numerous jet skis were seen close by at the time.
RNLI lifeguards dashed into the sea as eight other children became at risk of being swept away by a sudden and strong undercurrent, while the beach was crowded with thousands of families enjoying 23C sun during the May half-term holiday.
An 18-year-old swimmer is believed to have spotted the 12-year-old girl floating in the sea and helped recover her to the beach where she was given CPR.
The lifeless body of the 17-year-old boy was then spotted washed up close to the shoreline and lifeguards and an ambulance crew attempted to save him, before they were both airlifted to hospital.
In an update on Thursday, Dorset Police said that "early investigation indicates that there was no physical contact between a vessel and any swimmers at the time of the incident" but they "continue to investigate the circumstances".
The force said it received a report from paramedics at 4.32pm of people requiring assistance on the beach by the pier.
Witnesses reported their shock as they saw "loads of people rushing on to the beach" as two air ambulances, RNLI lifeguards and a lifeboat, South West Ambulance Service paramedics and Poole and Southbourne coastguard descended on the scene.
The beach and the nearby Pier Approach were cleared, with a major incident declared and a cordon set up.
Eeman Qamar, 33, from Southampton, was sat directly next to the pier on the beach with her 59-year-old mother and three-month-old baby for the 45 minutes before the incident and throughout it.
She told The Telegraph that "within seconds it just went from a normal fun day out at the beach to a very serious situation - tense, grave and sombre".
"At around 4.15pm we heard the lifeguards ringing the bells and they rushed up to us in a panic asking us to leave the area as an air ambulance was approaching. Within a couple of minutes I saw lots of people running from the pier side of the beach to the other side," she said.
"Then at around 4.30pm we saw lifeguards on jet skis and boats in the water, at least six big ambulances, doctor cars, RNLI and coastguard cars and lots of police, followed by the air ambulances which landed quickly about 20 minutes apart."
She said she "definitely did not see anybody jumping off the pier, nor did the people around me", but she did see "quite a few leisure jet skis in the water around the pier" which were "going around fast with people having a good time".
Rob Creech, the father of the 18-year-old swimmer who first came to the dying girl's aid, said: "My son was on the other side of the pier swimming with a few friends. All of a sudden there were a lot of people on the pier shouting and screaming that there was somebody in the water.
"He scooped her up to swim to shore... the emergency services took it from there."
Nicola Holton, 43, and Stuart Clark, 42, who were on the beach with their two children when the tragedy unfolded, said the beach became "a scene from a horror film" shortly after lifeguards announced there was "a dangerous riptide", having also told children several hours earlier to stop climbing on the pier.
Mr Clark said: "As we walked to the east side of the pier there was at least one but probably a few jet skiers going across the right side of the pier. It got really chaotic when they brought a young man back to shore on a lifeguard jet ski. It was obvious he wasn't alive."
Miss Holton said: "When we first spotted the swimmers in distress they were far out to the east side of the pier.
"A lifeguard ran into the water with a surfboard and it seemed to take ages for him to get to them... A few were taken to the lifeguard tent and then we saw the young man and girl brought out. We were packing up our things to leave as quickly as we could. It was just awful.
"There were loads of idiots ignoring lifeguard requests to get out of the water and clear the beach. People were running towards those having CPR filming on their phones."
Det Ch Supt 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."
There are yellow marker buoys 200m off the beach and water craft operating inside that area are restricted to speeds of six knots.
Kathryn Walton, from Oxford, was also on the scene with her family and described seeing lifeguards and "loads of people rushing on to the beach".
Bournemouth West MP Conor Burns said the tragedy was a "salutary lesson that our beaches and ocean can give much pleasure but danger is ever present".
"A dreadful event in circumstances when they were enjoying beautiful weather in our town. So sad," he wrote on Twitter. "Thanks to the lifeguards and the Air Ambulance who we can take for granted."
The investigation is being led by Dorset Police alongside the Marine Accident Investigation Branch and the Maritime and Coastguard Agency.
Tobias Ellwood, the Conservative MP for Bournemouth East, said: "This is a tragic incident. Our thoughts and prayers go out to the families and friends of the young girl and boy who lost their lives.
"Thank you to all the emergency services that responded so quickly."
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