Usher and Tameka Foster in February 2007. Credit: Mark Sullivan/WireImage

An investigation is now underway into Friday's tragic jet ski accident that left Usher's stepson brain dead.

According to ABC News, Jeffrey S. Hubbard, 38, was operating the jet ski when it collided with Kile Glover, 11, the son of Usher's ex-wife, Tameka Foster. Hubbard is a family friend of Usher and Foster.

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

Glover was riding an inner tube on Lake Lanier in Atlanta, GA on Friday with an unidentified 15-year-old girl. Both children were airlifted to Children's Healthcare of Atlanta at Egleston after the accident. Glover was hit in the head and registered no brain activity since he was admitted. The second victim reportedly suffered a broken arm and cut on her head.

Burgamy explained, "A pontoon was pulling the two adolescents on an inner tube, and the personal water craft went over the top of them."

Usher (real name: Usher Terry Raymond IV), 33, and Foster divorced after two years of marriage in 2009. The couple share sons Usher Raymond V, 4, and Naviyd, 3, in addition to Kile, Foster's son from a previous relationship.

Despite being engaged in a heated custody battle before the accident, Usher and Foster have put it aside to support each other. "Tameka and Usher don't get along, but in this circumstance Usher is coming to her side," a source told Us Weekly. "Usher practically raised Kile as his own. He's known him for seven years. It is so sad what happened." 

Another source told Us, "He loved Kile. This is a terrible, awful tragedy. Usher is torn up."