Prosecutors worked Tuesday to shore up the credibility of their chief cooperating witness in the Dewey & LeBoeuf criminal trial, Francis Canellas, following two days in which defense lawyers challenged testimony by Canellas that appeared to tie their clients to improper accounting adjustments at the firm.

Canellas, who was Dewey’s finance director before the firm collapsed in 2012, has been testifying for more than a week in the Manhattan district attorney’s case against former chairman Steven Davis, former executive director Stephen DiCarmine and former CFO Joel Sanders. Canellas’ testimony may be key to convicting the defendants, who are accused of duping lenders and investors about the firm’s finances. But during cross-examination, Canellas acknowledged that he hadn’t explicitly told his bosses that the firm was using improper accounting tricks.