According to Hollywood Reporter, a federal judge denied the studio access to legal documents supposedly outlining an agreement between the creators' estates not to strike any further copyright deals.
The materials are also alleged to contain details of how the Siegel and Shuster families are planning to divide their Superman assets once the Warner agreement has been terminated. The documents were at the centre of the studio's lawsuit against Siegel family attorney Marc Toberoff.
Warner legal representative Daniel Petrocelli challenged the notion that the papers are protected under attorney-client privilege, but U.S. Magistrate Judge Ralph Zaresky ruled in favour of Toberoff and his clients earlier this week.
In 2008, Joanne Siegel and daughter Laura Siegel Larson successfully recaptured half of the original copyright to Superman, sparking a legal backlash from Warner. The tactics behind the studio's retaliation were the subject of a letter penned by Joanne Siegel two weeks before her death.
The Shuster estate will have the opportunity to reclaim their percentage of the property's ownership in 2013.