Credit: ABC News

A family friend of R&B star Usher is under investigation after the singer's 11-year-old stepson was reportedly left brain dead after a jet ski accident in Georgia last week.

Jeffrey S. Hubbard, 38, a friend of Usher and his ex-wife, Tameka Foster, is at the center of the investigation. A July 8 collision on Georgia's Lake Lanier caused Kyle Glover, Foster's son with Atlanta TV executive Ryan Glover, and an unidentified 15-year-old girl to be airlifted to an Atlanta hospital, where Glover still remains.

"The accident is under investigation, and Hubbard is [under investigation] as the operator of the personal watercraft," Sgt. Mike Burgamy of the Georgia Department of Natural Resources told ABC News.

Burgamy described a pontoon "pulling the two adolescents on an inner tube" when a "personal water craft went over the top of them." Burgamy said that approximately 20 seconds after the accident occurred, Eric Isom, one of two department rangers who witnessed the incident, jumped into the water to assist the two kids. Glover was hit in the head and was unresponsive at the scene. He was found face up in water, and the unidentified girl was found face down.

The girl sustained a broken arm and a laceration on her head, according to ABC News affiliate WSB.

Burgamy said neither drugs nor alcohol are believed to have been a factor in the accident.

Usher, whose full name is Usher Raymond IV, and his ex-wife made headlines this year amid a bitter custody dispute over their two children. According to TMZ, this tragedy brought them together. TMZ said Usher chartered a plane for Foster, who was out of town at the time of the accident, so she could be by her son's side.

For Usher, the accident is the latest in a string of troubles. Last month, he had to call 911 and get a restraining order after a woman got into his gated community, camped out on his front stairs and rang his doorbell repeatedly. While now divorced from the mother of his children, Usher recently told ABC's Robin Roberts he takes fatherhood very seriously. "My steadfast belief is that a father is important in a child's life," he said.

ABC News Radio contributed to this report.