Scherer and others said Rothstein, who is serving a 50-year prison sentence, promised to come clean in hopes of one day getting out. Federal prosecutors have recommended an unspecified reduction in Rothstein's sentence, but only if he continues to co-operate.
Scherer quoted Rothstein as saying: "The only way I cannot die in prison is to tell the truth.''
Others implicated previously in the scheme include at least one executive at a branch of TD Bank, which has repeatedly denied wrongdoing but could be sued by investors based on Rothstein's testimony.
Former Rothstein associates have been charged with creating fake bank documents and fictitious websites that made it appear to investors that their money was safe.
Prosecutors are attending Rothstein's deposition. It is being held in private because Rothstein, 49, is part of the prison witness protection programme following his
assistance in fingering others linked to his own fraud and in the conviction of a South Florida man with organised crime ties.
A federal bankruptcy judge is holding a hearing today on an attempt by Scherer's firm to allow news media quick access to the transcripts. Otherwise it could be days or weeks before they are released.
Victims in the scheme have filed claims for about US$360 million in losses, with about US$100 million recovered so far.
Rothstein's homes, fancy cars, exotic jewellery and many other assets have been seized and sold to benefit investors.
Seven other Rothstein associates have been charged with crimes, but so far not the potential major players in the scheme. US Attorney Wifredo Ferrer said recently that more charges are expected in one of the biggest fraud cases in South Florida history.