KEEGO HARBOR, Mich. -

A 6-year-old girl who was in critical condition after being struck by a boat while riding in an inner tube Sunday at Sylvan Lake has died.

The girl's 11-year-old brother was killed when the boat hit the inner tube he and his two sisters were being pulled in. The children were being pulled by their father who was driving a jet ski. Another boater crossed their path, striking the inner tube.

Read back: Boy dies in Sylvan Lake boat crash, 2 sisters hospitalized

The children were rushed to the shore where several witnesses who claimed to be doctors and nurses tried to help them before EMS arrived.

The 11-year-old boy died from his injuries. The Oakland County Medical Examiner's Office has confirmed head and chest trauma as the cause of death.

His sisters, ages 6 and 10, were hospitalized in critical condition. The 10-year-old girl still is in critical condition.

Alcohol is not believed to be a factor in the accident.

The boater who hit the children is cooperating with investigators.

"I'll never forget the voice that was just, I mean, it was in terror yelling, 'Help! Police! Help!" said a witness.

Oakland County Sheriff Michael Bouchard said it was one of the worst accidents he has seen on the water in the county.

"You think as a parent what goes through your head. Losing one child is horrific. You can't imagine it. Having three children in that situation, I can't even get my mind around," said Bouchard.

The Mansour family and Beaumont Health System identify the children as Adriana Mansour, 10, Gabrielle Mansour, 6, and 11-year-old Alexander Monsour. The family is from West Bloomfield.

A vigil was held for the children at 8 p.m. Monday at the St. Thomas Chaldean Catholic Church grotto at 6900 Maple Road in West Bloomfield.

Watch: Vigil held for children killed in Sylvan Lake boat accident

Child killed, 2 injured on Sylvan Lake
Sylvan Lake is in Oakland County near Keego Harbor just southwest of Pontiac.