Mark Rucker, the highly regarded associate artistic director at American Conservatory Theater since 2010, was found dead in his San Francisco apartment Tuesday, officials said. He was 56.

The cause of Mr. Rucker’s death has not been released and is still under investigation, a San Francisco medical examiner’s office spokesman said Wednesday. Police said the death was not being investigated as a criminal matter.

In a statement, ACT Artistic Director Carey Perloff said, “The ACT community is mourning the passing of our beloved Mark Rucker. He was a treasured friend, an inspirational colleague, a brilliant artist and an extraordinary mentor to generations of young theater artists. We will miss him terribly. Our thoughts and prayers are with Mark’s family and his friends around the globe.”

Perloff added in an interview, “He left an incredible legacy here. He was a lively, passionate director who was really happiest in the rehearsal room.”

A regular contributor to every ACT season, Mr. Rucker most recently directed Anne Washburn’s “The Simpsons”-esque dark comedy, “Mr. Burns, a Post-Electric Play” last spring. He was next scheduled to stage Eugene O’Neill’s “Ah, Wilderness!” for ACT in October.

Raised in Newport Beach in Southern California, Mr. Rucker began directing plays in third grade and studied the craft as an undergraduate at UCLA. He started directing professionally at Shakespeare Santa Cruz in the late 1980s, went to Yale for his master’s of fine arts degree in directing and established an enviable career as a freelance director in the 1990s.

Though he worked regularly at Orange County’s South Coast Repertory, he was in demand throughout the country, working with many of the most prestigious companies — the La Jolla Playhouse, Old Globe, Oregon Shakespeare Festival, Arena Stage and Intiman Theatre — as well as smaller, adventurous groups such as Culture Clash.

He also achieved something of a cult following as the director of the 2003 film “Die, Mommie, Die!,” based on the camp classic play by Charles Busch. The film received a special jury prize at Sundance.

His work was seen frequently in the Bay Area at California Shakespeare Theater, where he last directed Noel Coward’s “Blithe Spirit” three years ago, as well as at Berkeley Rep, the Magic Theatre, San Francisco Playhouse and Campo Santo. In 2014, he returned to Santa Cruz to help the new Santa Cruz Shakespeare company, the successor to Shakespeare Santa Cruz, mount its first season, directing “As You Like It.”

His principal home, however, was at ACT, where Mr. Rucker made his debut with Amy Freed’s Shakespearean comedy “The Beard of Avon” in 2002. Besides directing many shows there, including such new works as Amy Herzog’s “4000 Miles” and Jordan Harrison’s “Maple and Vine,” he was known as an avid teacher in ACT’s MFA program and for building bridges between ACT and the rest of the theater community, including work with Intersection for the Arts.

He was one of the principal arrangers of the three-theater collaboration that brought notable young playwright Tarell Alvin McCraney’s “The Brother/Sister Plays” trilogy to the Bay Area in 2010, at Marin Theatre Company, ACT and the Magic — and directed one of the plays, “Marcus, or the Secret of Sweet.”

Funeral services have not been announced.

San Francisco Chronicle

staff writer Henry K. Lee contributed to this report.

Robert Hurwitt is The San Francisco Chronicle’s theater critic. E-mail: rhurwitt@sfchronicle.com Twitter: @RobertHurwitt