In a major shift of policy, the Justice Department, once known for taking down giant corporations, including the accounting firm Arthur Andersen, has put off prosecuting more than 50 companies suspected of wrongdoing over the last three years. Instead, many companies, from boutique outfits to immense corporations like American Express, have avoided the cost and stigma of defending themselves against criminal charges with a so-called deferred prosecution agreement, which allows the government to collect fines and appoint an outside monitor to impose internal reforms without going through a trial. In many cases, the name of the monitor and the details of the agreement are kept secret. Deferred prosecutions have become a favorite tool of the Bush administration. But some legal experts now wonder if the policy shift has led companies, in particular financial institutions now under investigation for their roles in the subprime mortgage debacle, to test the limits of corporate anti-fraud laws. Firms have readily agreed to the deferred prosecutions, said Vikramaditya S. Khanna, a law professor at the University of Michigan who has studied their use, because clearly it avoids a bigger headache for them." Some lawyers suggest that companies may be willing to take more risks because they know that, if they are caught, the chances of getting a deferred prosecution are good. "Some companies may bear the risk" of legally questionable business practices if they believe they can cut a deal to defer their prosecution indefinitely, Mr. Khanna said. Legal experts say the tactic may have sent the wrong signal to corporations - the promise, in effect, of a get-out-of-jail-free card.