16/07/2013 16:39 BST | Updated 22/05/2015 10:12 BST

Boy, 11, And Sister, 6, Killed In Water Tube Accident

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An 11-year-old boy and his six-year-old sister have been killed in a boating accident that left another sibling fighting for her life.

Alex and Gabrielle Mansour and their 10-year-old sister, Adriana, were in a tube being pulled behind their father, John Mansour, who was riding a jet ski.

A boater swerved to avoid the jet ski but failed to see the tube, striking the three children head on.

The horrific accident happened on Sylvan Lake, Michigan, US on Sunday.

Both the father and the boat operator frantically tried to help the kids, pulling them out of the water and on to their crafts before rushing to shore.

But despite the efforts of paramedics and doctors, 11-year-old Alex died of injuries to his chest shortly after the crash.

His two sisters were transported to hospital where Gabrielle, six, died on Monday and Adriana, 10, remains in critical condition.

An autopsy has ruled the deaths accidental and caused by 'multiple blunt force injuries'.

Oakland County Sheriff Mike Bouchard called the horrific accident 'one of the worst boating tragedies that we've seen in a long time.'

"We're going to do a complete and thorough investigation," he said.

"But in the end it won't make anybody feel any better... And regardless of the results, it's not going to the heal the wounds that this has created; it's so very sad."

Resident Cindee Rosenthal, who saw the tragedy unfold, said the children's father was hysterical after the collision.

"I looked up and the tube was floating and the three children were floating in the water with their vests on," she told reporters.

"The man on the jet ski had his hands on his head yelling, 'Oh, my God, oh, my God, someone help me!' And then I heard someone yell 'Call 911!'

"And other boats were heading toward the scene. (There was) no noise from the children - it was all adults yelling. And of course the father was hysterical."