LOS ANGELES -- A medical mystery was solved Friday: T.R. Knight won't be back on "Grey's Anatomy" next season.

"Leaving 'Grey's Anatomy' was not an easy decision for me to make. I am extremely grateful to have had the opportunity to play this character and will miss my fellow cast and crew very much," Knight said in a statement.

He also thanked fans for their support, but conspicuously didn't mention series creator and executive producer Shonda Rhimes.

Knight, who played Dr. George O'Malley, had seen his role diminished this season and asked to be let out of a contract that still had three years to go. In the season finale, his character had been injured -- fatally, it appeared -- as he saved a woman from a bus collision.

In a statement from ABC announcing Knight's much-rumoured departure, Rhimes wished Knight "the best in his future endeavours" on behalf of the show's "family."

"He is an incredibly talented actor and a person whose strength of character is admired by all of us," Rhimes said.

Katherine Heigl's character, Dr. Izzie Stevens, also appeared to have an uncertain future after brain surgery in the May finale. But Heigl will return to the show, ABC said Friday.

Heigl had previously expressed discontent with the writing for her character and, with steady film roles, was considered a candidate for departure. In March, however, she said she was ready to stay and the decision rested with producers.

Although "Grey's Anatomy" features an ensemble cast that includes stars Ellen Pompeo and Patrick Dempsey, Heigl is a key part of the top 10 show.

Knight, who has a theatre background, is said to be weighing stage projects as well as films for his post-"Grey's" life.

He was part of a tumultuous chapter in the history of the show, which debuted in 2005. In 2007, co-star Isaiah Washington was dropped from "Grey's Anatomy" after using an anti-gay epithet backstage at the Golden Globe Awards while denying he'd used it previously on the set against Knight, who is gay.