Usher's stepson remembered by Justin Bieber, Russell Simmons

(CBS/AP) Celebrities are sending their condolences to the family of Kile Glover, the stepson of pop star Usher who died Saturday morning, two weeks after the being critically injured in a boating accident.

The 11-year-old boy was a son of Usher's ex-wife Tameka Foster. His father is Atlanta TV executive Ryan Glover. Russell Simmons and Justin Bieber were among the stars who voiced their prayers on Twitter:

It's unclear what was the direct cause of Kile's death. Both E! News and the Los Angeles Times had reported that he was taken off life support, but a family friend later told WSB-TV in Atlanta that Kile died of heart failure.

Willie A. Watkins funeral home in Atlanta confirmed Saturday it was handling funeral arrangements for Kile.

"It's very, very, very difficult time right now," Foster's friend, producer Jodi Gomes, told E! News. "As a mother, I'm devastated for my friend. Tameka is holding up strong. She is very faithful and puts all her trust in the Lord, truly."

The boy was run over July 6 by a personal watercraft on Lake Lanier, according to the Georgia Department of Natural Resources. He had been hospitalized with a major brain injury. Lake Lanier is about 40 miles northeast of Atlanta.

Jeffrey Hubbard, a 38-year-old family friend, was driving the personal watercraft. Authorities continue to investigate the accident but have said that alcohol wasn't involved.

Before the incident, Usher, whose full name is Usher Raymond, had been in a legal battle with his ex-wife arising from a custody fight over the two sons they have together, ages 3 and 4.