Facing allegations that it aided fraud at the now-defunct Aequitas Securities LLC, Sidley Austin has argued in court papers filed in Oregon federal court that if the firm is ultimately held liable, a group of investment advisory businesses should share in the blame.

Sidley filed a third-party complaint Monday as part of its defense in a proposed investor class action. The investors allege that Aequitas ran something similar to a Ponzi scheme, relying on funds coming in from new investors to meet its obligations to repay others. They also claim that Sidley, along with prominent auditing firms and others, contributed to a string of misconduct and deception on the part of Aequitas when they did work for the now insolvent company.