Praised documentary filmmaker Bill Siegel, known for The Trials Of Muhammad Ali and The Weather Underground among others, has died. He was 55.

Chicago-based Kartemquin Films announced Siegel’s death on its website Tuesday. “Bill was an erudite, passionate and intelligent activist and independent thinker as well as being a gifted filmmaker and cherished friend,” the statement read. “He has been a valued member of the Kartemquin community since he first worked as a researcher on Hoop Dreams (1994). We will miss him greatly.” The cause of death is not immediately known.

Siegel won an Emmy for his 2013 documentary, The Trials of Muhammad Ali, and he shared an Oscar nomination in the best documentary category for The Weather Underground.

The Trials of Muhammad Ali documented the heyday of boxer Muhammad Ali’s career, with special focus on his conversion to Islam and his refusal to fight in the Vietnam War. In addition to the Emmy win, the film won an award for Best Use of News Footage from the International Documentary Association. The Trials of Muhammad Ali debuted at the Tribeca Film Festival and later aired on PBS.

Siegel and co-director Sam Green were Oscar-nominated in 2004 for The Weather Underground, a groundbreaking portrait that chronicled the rise and fall of the American radical organization. The film, which originally premiered at the 2003 Sundance Film Festival, constructed a linear narrative of the organization, using archival footage as well as interviews with its members.

Siegel also worked for many years at the Great Books Foundation, where he trained teachers in inquiry-based learning and offered consulting, video production and training services to schools, corporations, non-profits and community organizations across the country.