Novelist Michael Chabon has apologized for “enabling” producer Scott Rudin’s alleged abusive treatment of his employees.

In an essay titled “Apology of a Rudin Apologist” published on Medium, Chabon responded to recent reports that appeared in The Hollywood Reporter and Vulture in which former staffers accuse Rudin of severe abuse, including, in some cases, physical assault.

“I heard stories of Scott’s tantrums and vindictiveness, but not of smashed hands and people pushed out of moving cars,” Chabon wrote. “But I knew enough.”

Chabon and Rudin collaborated on and off for 20 years, most notably on the Oscar-nominated 2000 film adaptation of Chabon’s novel Wonder Boys. They also worked on an adaptation of Chabon’s Pulitzer Prize–winning novel, The Amazing Adventures of Kavalier and Clay, which was never made.

Chabon said he witnessed Rudin treat staff with “a careful, even surgical contempt, like a torturer trained to cause injuries that leave no visible marks.” He also said he witnessed Rudin throw a pencil at an assistant when the producer didn’t realize that Chabon was present.

The novelist said he cut ties with Rudin in 2015, after he learned that Rudin had been insulting his wife, the author Ayelet Waldman, behind his back.

“I’m ashamed,” Chabon wrote. “I regret, and I want to apologize for, my part in enabling Scott Rudin’s abuse, simply by standing by, saying nothing, looking the other way.”

Michael Schaub is a Texas-based journalist and regular contributor to NPR.